That error message is displayed when one of the MNet peer servers sends a request to the other and the returned response does not seem to be encrypted. It's pretty tricky to debug (as any other problem in MNet ...). The first thing to look at is the admin page that checks the server environment (PHP extensions, versions etc). Make sure that both server report all green, no missing SSL or XML extensions etc.
Moodle networking (MNet)
Moodle 1.9 to Moodle 2.2 "payload not encrypted"
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