This seems to be a perennial issue, that apparently never got resolved fully.
I am currently using latest update of ST 3.0.17 and GitBlit as a remote host, but have been experiencing this problem on and off with many different versions. OS is Windows 10.
Our office policy is to change passwords fairly frequently. every time this happens, I am unable to access remote repositories from SourceTree, and there is no way to update credentials from SourceTree. I have tried many solutions suggested by the community, none of them worked. Not even deleting psswd file in users.
At random, I would get a chance to type in password. It does not work, and yes, I have confirmed that I can log in into my repository on GitBlit.
One way or the other, this has been happening a lot, maybe it's time to add authentication update option to the repository settings?