How to fix Push Failed unexpected HTTP status code: 500 GitKraken

I had exactly the same issue - git push from command line works, but any pushes from gitkraken resulted in a 500 error. We use a BitBucket server, and authentication has always been with username/passwords that match our Windows domain logon credentials. Whilst trying to investigate I tried using SSH authentication and BitBucket Personal Access Tokens, but neither made any difference.

I've found that I don't get an issue if I push ONE change at a time, and only see the issue when multiple commits are being pushed. I had thought I had solved the problem by clearing out my cache and re-entering details but this only appeared to work because I tested my changes by pushing one more change from local to origin. So, I haven't got a fix, only a workaround - push after any local commit.