I only use source tree for one of one project that has mainly text file (sql scripts) and now an excel file. For other code projects I use the git command line.
I got an error that said something like "Commit was successful though push failed" which makes sense if I needed to pull first. Only thing was there was nothing to pull after fetching and going back to commit there is now no changes. My colleague who uses the git cmd line says the symptoms are like an rebase hard as all changes since my last commit yesterday failed. The other interesting thing was that I was the last person to commit yesterday and there was no reason to pull before pushing!
I believe this might be to do with something committing the excel being open (file closed though excel 2016 still open) with the file that is in source control as this is only an file I started to commit recently.
I regularly update source tree and was running the latest version at the time of this error a short time ago.