Make sure you're using the svn+ssh:// protocol.
Examine the .bashrc file on your user's account in the SSH host, and make sure that the command mesg y (or mesg n) isn't in the file. If it is, comment it out.
Actually, there are plenty of other causes: anything that generates an error has to be fixed, since the SSH connection doesn't know how to handle it. Any error causes the SSH link to close.
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