The Beyond Compare executable always changes back to the incorrect one causing the diff to fail on more than one file.
Sourcetree does not appear to save the path correctly and I have to modify it almost every time I relaunch it.
Sourcetree should use BComp.exe and not BCompare.exe for the Diff. This is the same as the Merge tool, which does appear to work properly.
Can you check you are running v3.0.15 this loss of settings should be fixed in that release.
We are looking into the use of BComp.exe vs BCompae.exe as it appears to work correctly for us and looking at https://www.scootersoftware.com/v4help/index.html?command_line_reference.html other than opening an instance per comparison vs opening a tab per comparison they don't appear to operate differently. However if you have additional information please let us know.
Additionally, closing SourceTree and reopening it returns the Beyond Compare diff settings back to BCompare.exe (my original problem reported here).
I believe it relates to the way BCompare.exe only opens one window and BComp.exe pushes to the same open window: https://www.scootersoftware.com/v4help/index.html?calling_bc_from_other_applications.html