Add a troubleshooting note to the FAQ

docs/faq.rst

@@ -712,6 +712,15 @@ the nonce is deleted or if you suspect it may have been tampered with. See :ref:
 Common issues
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+/path/to/repo is not a valid repository. Check repo config.
+-----------------------------------------------------------
+
+There can be many causes of this error. E.g. you have incorrectly specified the repository path.
+
+You will also get this error if you try to access a repository that uses the argon2 key algorithm using an old version of borg.
+We recommend upgrading to the latest stable version and trying again. We are sorry. We should have thought abount forward
+compatibility and implemented a more helpful error message.
+
 Why does Borg extract hang after some time?
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