I get this Authenticate window every so often popping up somewhere, asking me for a password. The username is readonly, cannot be changed, and is incorrect for the server it requires a password for.

It doesn't matter if I type in bollocks or my actual password, or just cancel the window. The window will always pop up a few hours later. It also doesn't matter if I set up ST to use my system Git (2.4.5) or the embedded Git (1.9.5).

What's more, everything just works fine. I can use the very same server in Sourcetree absolutely fine.

In short:
1) The username cannot be changed
2) Even if it could, this popup will reappear no matter what