Ever since SourceTree 1.9 I've been having trouble with the OS X Keychain not remembering that I've allowed SourceTree to have permanent access to the stored credentials for our Stash server. I have to keep hitting "Allow" even though I've clicked "Always Allow," as well.
It's possible it may have coincided with Mac OS X 10.9.3 update, as well (that just came out, didn't it?)
Here's the popup message I get (every time SourceTree connects to Stash)
I've even tried checking "Allow all applications to access this item" but it keeps getting recent to this (and this SHOULD work.)
Here's the "Internet Password" entry in OS X Keychain
Funny how I'm still having this problem in 2020, I blame this on Apple though as I've experienced this same problem in one way or another with all of their products. They demand you link up with other services, or they punish you with alerts and popups non-stop till you are shaking with rage. None of the above solutions worked for me but I found one that did-
Remove the Atlassian/BitBucket association from SourceTree. Preferences>Accounts>RemoveAccount