sourcetree, Error: remote: Invalid username or password. fatal: Authentication failed for 'https://github.com/

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. Delete AppData\Local\Atlassian\SourceTree\passwd 
3. Restart SourceTree
4. Hurray!!!