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)
Keychain Popup Error
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
https://www.dropbox.com/s/cw5209r071xgohv/Keychain.png