if I try to start a server on OSX via SSH, an error occurs:
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sh: line1: rdiff-backup: command not found
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rdiff-backup _is_ in the PATH and I'm able to start other commands
successfully.
Well, it's clearly not in the PATH. :-)
Where did you install it? Also, when a program is invoked via SSH, the
PATH gets setup differently than when you invoke an interactive shell. I
can't remember the specific details, however.