I found the answer here, it is a known bug for version 2.1.8+
The following steps fixed it for me:
Tools > Options > Git > Update Embedded Git
Also pushing with the command line works.
EDIT
Also adding @Latisha’s answer:
1. Do the above.
2. Close Sourcetree
3. Delete AppData\Local\Atlassian\SourceTree\passwd
4. Restart SourceTree