Sourcetree/Mac was working just fine until it asked to be updated. Like a fool, I thought nothing of it and updated. Now I have this version:
When I try to open a repository, or select a file to do a commit on an already open repo, it crashes:

The error I see is:
Process: Sourcetree [89885]
Path: /Applications/Sourcetree.app/Contents/MacOS/Sourcetree
Identifier: com.torusknot.SourceTreeNotMAS
Version: 4.1.3 (239)
Code Type: X86-64 (Native)
Parent Process: ??? [1]
Responsible: Sourcetree [89885]
...
Date/Time: 2021-08-19 12:07:51.363 +0100
OS Version: Mac OS X 10.15.7 (19H1217)
Report Version: 12
...
System Integrity Protection: enabled
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_BAD_INSTRUCTION (SIGILL)
Exception Codes: 0x0000000000000001, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Termination Signal: Illegal instruction: 4
Termination Reason: Namespace SIGNAL, Code 0x4
Terminating Process: exc handler [89885]
Application Specific Information:
Crashing on exception: Couldn't posix_spawn: error 9
Application Specific Backtrace 1:
0 CoreFoundation 0x00007fff31347627 __exceptionPreprocess + 250
1 libobjc.A.dylib 0x00007fff6a2275bf objc_exception_throw + 48
2 Foundation 0x00007fff33a0a971 -[NSConcreteTask launchWithDictionary
] + 4993
3 Sourcetree 0x0000000106cb4002 Sourcetree + 860162
4 Sourcetree 0x0000000106cb1875 Sourcetree + 850037
5 Sourcetree 0x0000000106cd5b88 Sourcetree + 998280
6 AppKit 0x00007fff2e7351de -[NSWindowController _windowDidLoad] + 624
7 AppKit 0x00007fff2e730f94 -[NSWindowController window] + 110
8 AppKit 0x00007fff2e7b3a62 -[NSWindowController showWindow:] + 36
9 Sourcetree 0x0000000106cd5b54 Sourcetree + 998228
10 Sourcetree 0x0000000106d0230a Sourcetree + 1180426
11 Sourcetree 0x0000000106c10506 Sourcetree + 189702
12 Sourcetree 0x0000000106c10681 Sourcetree + 190081
13 Sourcetree 0x0000000106c1080c Sourcetree + 190476
14 Sourcetree 0x0000000106c109cf Sourcetree + 190927
15 Sourcetree 0x0000000106db3a24 Sourcetree + 1907236
16 Sourcetree 0x0000000106db3aea Sourcetree + 1907434
17 AppKit 0x00007fff2e77e7a7 -[NSApplication(NSResponder) sendAction:to:from:] + 299
18 AppKit 0x00007fff2e77e642 -[NSControl sendAction:to:] + 86
19 AppKit 0x00007fff2e8decd6 -[NSTableView _sendAction:to:row:column:] + 114
20 AppKit 0x00007fff2e8dd266 -[NSTableView mouseDown:] + 6785
21 AppKit 0x00007fff2e8db5f5 -[NSOutlineView mouseDown:] + 73
22 AppKit 0x00007fff2e7775f0 -[NSWindow(NSEventRouting) _handleMouseDownEvent:isDelayedEvent:] + 4914
23 AppKit 0x00007fff2e6e1e21 -[NSWindow(NSEventRouting) _reallySendEvent:isDelayedEvent:] + 2612
24 AppKit 0x00007fff2e6e11c9 -[NSWindow(NSEventRouting) sendEvent:] + 349
25 AppKit 0x00007fff2e6df554 -[NSApplication(NSEvent) sendEvent:] + 352
26 AppKit 0x00007fff2e52c5bf -[NSApplication run] + 707
27 AppKit 0x00007fff2e4fe396 NSApplicationMain + 777
28 libdyld.dylib 0x00007fff6b3cfcc9 start + 1
29 ??? 0x0000000000000001 0x0 + 1
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 com.apple.AppKit 0x00007fff2e91e373 -[NSApplication _crashOnException:] + 106
1 com.apple.AppKit 0x00007fff2e91e256 -[NSApplication reportException:] + 901
2 com.apple.AppKit 0x00007fff2e52c640 -[NSApplication run] + 836
3 com.apple.AppKit 0x00007fff2e4fe396 NSApplicationMain + 777
4 libdyld.dylib 0x00007fff6b3cfcc9 start + 1
Thread 1:: com.apple.NSEventThread
0 libsystem_kernel.dylib 0x00007fff6b510dfa mach_msg_trap + 10
1 libsystem_kernel.dylib 0x00007fff6b511170 mach_msg + 60
2 com.apple.CoreFoundation 0x00007fff312cb985 __CFRunLoopServiceMachPort + 247
3 com.apple.CoreFoundation 0x00007fff312ca452 __CFRunLoopRun + 1319
4 com.apple.CoreFoundation 0x00007fff312c98ce CFRunLoopRunSpecific + 462
5 com.apple.AppKit 0x00007fff2e6dd954 _NSEventThread + 132
6 libsystem_pthread.dylib 0x00007fff6b5d4109 _pthread_start + 148
7 libsystem_pthread.dylib 0x00007fff6b5cfb8b thread_start + 15
Thread 2:
0 libsystem_pthread.dylib 0x00007fff6b5cfb68 start_wqthread + 0
Thread 3:: Dispatch queue: com.apple.root.user-initiated-qos
0 libsystem_kernel.dylib 0x00007fff6b51181e read + 10
1 com.apple.Foundation 0x00007fff33931b9e _NSReadFromFileDescriptorWithProgress + 232
2 com.apple.Foundation 0x00007fff33951da9 -[NSConcreteFileHandle readDataOfLength:] + 404
3 com.torusknot.SourceTreeNotMAS 0x0000000106c2a252 0x106be2000 + 295506
4 com.torusknot.SourceTreeNotMAS 0x0000000106c2b989 0x106be2000 + 301449
5 com.torusknot.SourceTreeNotMAS 0x0000000106c2a113 0x106be2000 + 295187
6 com.torusknot.SourceTreeNotMAS 0x0000000106d04dea 0x106be2000 + 1191402
7 com.torusknot.SourceTreeNotMAS 0x0000000106ceb503 0x106be2000 + 1086723
8 libdispatch.dylib 0x00007fff6b3756c4 _dispatch_call_block_and_release + 12
9 libdispatch.dylib 0x00007fff6b376658 _dispatch_client_callout + 8
10 libdispatch.dylib 0x00007fff6b384bfc _dispatch_root_queue_drain + 1003
11 libdispatch.dylib 0x00007fff6b385097 _dispatch_worker_thread2 + 92
12 libsystem_pthread.dylib 0x00007fff6b5d09f7 _pthread_wqthread + 220
13 libsystem_pthread.dylib 0x00007fff6b5cfb77 start_wqthread + 15
Thread 4:: Dispatch queue: com.apple.root.default-qos
0 libsystem_kernel.dylib 0x00007fff6b51255e __ulock_wait + 10
1 libdispatch.dylib 0x00007fff6b376e29 _dlock_wait + 44
2 libdispatch.dylib 0x00007fff6b3770b7 _dispatch_group_wait_slow + 43
3 com.torusknot.SourceTreeNotMAS 0x0000000106cea3a2 0x106be2000 + 1082274
4 com.torusknot.SourceTreeNotMAS 0x0000000106cec2ac 0x106be2000 + 1090220
5 libdispatch.dylib 0x00007fff6b3756c4 _dispatch_call_block_and_release + 12
6 libdispatch.dylib 0x00007fff6b376658 _dispatch_client_callout + 8
7 libdispatch.dylib 0x00007fff6b3784b0 _dispatch_queue_override_invoke + 763
8 libdispatch.dylib 0x00007fff6b384957 _dispatch_root_queue_drain + 326
9 libdispatch.dylib 0x00007fff6b385097 _dispatch_worker_thread2 + 92
10 libsystem_pthread.dylib 0x00007fff6b5d09f7 _pthread_wqthread + 220
11 libsystem_pthread.dylib 0x00007fff6b5cfb77 start_wqthread + 15
Thread 5:
0 libsystem_pthread.dylib 0x00007fff6b5cfb68 start_wqthread + 0
Thread 6:
0 libsystem_pthread.dylib 0x00007fff6b5cfb68 start_wqthread + 0
Thread 7:: Dispatch queue: com.apple.root.default-qos
0 libsystem_kernel.dylib 0x00007fff6b51255e __ulock_wait + 10
1 libdispatch.dylib 0x00007fff6b376e29 _dlock_wait + 44
2 libdispatch.dylib 0x00007fff6b3770b7 _dispatch_group_wait_slow + 43
3 com.torusknot.SourceTreeNotMAS 0x0000000106cc7b78 0x106be2000 + 940920
4 libdispatch.dylib 0x00007fff6b3756c4 _dispatch_call_block_and_release + 12
5 libdispatch.dylib 0x00007fff6b376658 _dispatch_client_callout + 8
6 libdispatch.dylib 0x00007fff6b3784b0 _dispatch_queue_override_invoke + 763
7 libdispatch.dylib 0x00007fff6b384957 _dispatch_root_queue_drain + 326
8 libdispatch.dylib 0x00007fff6b385097 _dispatch_worker_thread2 + 92
9 libsystem_pthread.dylib 0x00007fff6b5d09f7 _pthread_wqthread + 220
10 libsystem_pthread.dylib 0x00007fff6b5cfb77 start_wqthread + 15
Thread 8:: Dispatch queue: com.apple.root.default-qos
0 libsystem_kernel.dylib 0x00007fff6b510dfa mach_msg_trap + 10
1 libsystem_kernel.dylib 0x00007fff6b511170 mach_msg + 60
2 com.apple.CoreFoundation 0x00007fff312cb985 __CFRunLoopServiceMachPort + 247
3 com.apple.CoreFoundation 0x00007fff312ca452 __CFRunLoopRun + 1319
4 com.apple.CoreFoundation 0x00007fff312c98ce CFRunLoopRunSpecific + 462
5 com.apple.Foundation 0x00007fff33a19200 -[NSConcreteTask waitUntilExit] + 325
6 com.torusknot.SourceTreeNotMAS 0x0000000106c2b9b0 0x106be2000 + 301488
7 com.torusknot.SourceTreeNotMAS 0x0000000106c2a113 0x106be2000 + 295187
8 com.torusknot.SourceTreeNotMAS 0x0000000106d15e18 0x106be2000 + 1261080
9 com.torusknot.SourceTreeNotMAS 0x0000000106cc7dc0 0x106be2000 + 941504
10 libdispatch.dylib 0x00007fff6b3756c4 _dispatch_call_block_and_release + 12
11 libdispatch.dylib 0x00007fff6b376658 _dispatch_client_callout + 8
12 libdispatch.dylib 0x00007fff6b3785b0 _dispatch_queue_override_invoke + 1019
13 libdispatch.dylib 0x00007fff6b384957 _dispatch_root_queue_drain + 326
14 libdispatch.dylib 0x00007fff6b385097 _dispatch_worker_thread2 + 92
15 libsystem_pthread.dylib 0x00007fff6b5d09f7 _pthread_wqthread + 220
16 libsystem_pthread.dylib 0x00007fff6b5cfb77 start_wqthread + 15
Thread 9:
0 libsystem_pthread.dylib 0x00007fff6b5cfb68 start_wqthread + 0
Thread 10:
0 libsystem_pthread.dylib 0x00007fff6b5cfb68 start_wqthread + 0
Thread 11:: JavaScriptCore bmalloc scavenger
0 libsystem_kernel.dylib 0x00007fff6b513882 __psynch_cvwait + 10
1 libsystem_pthread.dylib 0x00007fff6b5d4425 _pthread_cond_wait + 698
2 libc++.1.dylib 0x00007fff686a2623 std::__1::condition_variable::__do_timed_wait(std::__1::unique_lock<std::__1::mutex>&, std::__1::chrono::time_point<std::__1::chrono::system_clock, std::__1::chrono::duration<long long, std::__1::ratio<1l, 1000000000l> > >) + 93
3 com.apple.JavaScriptCore 0x00007fff35acdc35 bmalloc::Scavenger::threadRunLoop() + 741
4 com.apple.JavaScriptCore 0x00007fff35acd649 bmalloc::Scavenger::threadEntryPoint(bmalloc::Scavenger*) + 9
5 com.apple.JavaScriptCore 0x00007fff35acfd27 void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(bmalloc::Scavenger*), bmalloc::Scavenger*> >(void*) + 39
6 libsystem_pthread.dylib 0x00007fff6b5d4109 _pthread_start + 148
7 libsystem_pthread.dylib 0x00007fff6b5cfb8b thread_start + 15
Thread 12:: com.apple.coreanimation.render-server
0 libsystem_kernel.dylib 0x00007fff6b510dfa mach_msg_trap + 10
1 libsystem_kernel.dylib 0x00007fff6b511170 mach_msg + 60
2 com.apple.QuartzCore 0x00007fff3ce00c8e CA::Render::Server::server_thread(void*) + 496
3 com.apple.QuartzCore 0x00007fff3ce00a97 thread_fun(void*) + 25
4 libsystem_pthread.dylib 0x00007fff6b5d4109 _pthread_start + 148
5 libsystem_pthread.dylib 0x00007fff6b5cfb8b thread_start + 15
Thread 13:
0 libsystem_pthread.dylib 0x00007fff6b5cfb68 start_wqthread + 0
Thread 0 crashed with X86 Thread State (64-bit):
rax: 0x00007f940d945c00 rbx: 0x00006000039d81e0 rcx: 0x944ab4e545030082 rdx: 0x0000000000000001
rdi: 0x00007ffee901d510 rsi: 0x0000000000000a9d rbp: 0x00007ffee901d980 rsp: 0x00007ffee901d970
r8: 0x0000000000000a00 r9: 0x00007f940d94669d r10: 0x000000000000009d r11: 0x00007ffee901d14a
r12: 0x00006000039d81e0 r13: 0x00007fff6a21a800 r14: 0x00007fff6a21a800 r15: 0x0000600000150000
rip: 0x00007fff2e91e373 rfl: 0x0000000000010202 cr2: 0x00007fa077088b78
Logical CPU: 4
Error Code: 0x00000000
Trap Number: 6
...
External Modification Summary:
Calls made by other processes targeting this process:
task_for_pid: 1
thread_create: 0
thread_set_state: 0
Calls made by this process:
task_for_pid: 0
thread_create: 0
thread_set_state: 0
Calls made by all processes on this machine:
task_for_pid: 1431209
thread_create: 0
thread_set_state: 0
VM Region Summary:
ReadOnly portion of Libraries: Total=777.5M resident=0K(0%) swapped_out_or_unallocated=777.5M(100%)
Writable regions: Total=692.8M written=0K(0%) resident=0K(0%) swapped_out=0K(0%) unallocated=692.8M(100%)
...
Model: MacBookPro11,3, BootROM 431.0.0.0.0, 4 processors, Quad-Core Intel Core i7, 2.3 GHz, 16 GB, SMC 2.19f12
...
I uninstalled and reinstalled, but the behaviour continues.