I deleted the passwd file in C:\Users\YOUR_USER\AppData\Local\Atlassian\SourceTree and also deleted any passwords for Bitbucket in the Windows Credential manager (Control Panel\User Accounts\Credential Manager) -> Windows Credentials
Next, I also checed the repository settings in the repo where the error occurs (cog symbol in the top right of Sourcetree -> click "Edit" on the remote repository path.
(After changing to look like other repository URLs, I could fetch with no problems)