I've been using SourceTree for several years. Recently, when I open it up and try to do any operation that requires communicating with Bitbucket, it pops up a login window asking for username and password. No matter what I enter there, it always fails.

If I open

Tools > Options > Authentication > (Bitbucket account) > Edit

it always shows "Authentication OK". Then, I might need to click "Refresh OAuth Token" or not (not 100% sure), but after this it will work again for a while.

I've tried everything I could think of, but I can't permanentely fix this.

Something odd I noticed, is that I see many icons in the tray area for the "Pageant (PuTTY authentication agent)" and as soon as I hover them they disappear (crashed?).