whitespaces in the remote path field should be trimmed automatically by SourceTree. If you inadvertently put some at the end/beginning (e.g. through copy/paste) you'll get an error like this:

Pushing to ssh://someuser@example.com/repo.gitfatal: '/www/htdocs/repo.git ' does not appear to be a git repositoryfatal: Could not read from remote repository.

it's hard to see the whitespace at the end of the path.