Doing a git clone/pull/push command might result in the following error message:
fatal: protocol error: bad line length character
fetch-pack from 'ssh://firstname.lastname@example.org/home/bram/Filesystem' failed.
This is rather cryptic and it took some imagination to see where the problem was. The user bram is restricted to only one login on that machine. In this case, there was already a SSH session with that user on that machine, so no room anymore for a git operation. Closing down that session made git work again.
There may be other causes than this, but when you see this error, try logging in with the ssh command and see if that succeeds.