Have you registered the server with AD? As in - you see it in AD Users & Computers? The message might be referring to your system not being found/registered. Sometimes this is because the DNS name for the server differs from the AD domain name.
Another thing to keep an eye on is the clock. If the clocks are to far out of sync lots of things break ...
This seems to be the culprit:
error Message is Server not found in Kerberos database
sname is cifs/***********@***********
The first question to ask is if the failing server has appropriate computer account in AD (and the fact it has sssd configured doesn't necessarily imply that depending on HOW sssd is configured).
Assuming this is present you may ...