For years, my node had similar behavior, causing various kinds of intermittent issues. The problem was solved when I added an iptables rule permitting inbound IPENCAP:

Ok in your case it was a configuration error?

Of course we should make sure that there is no error outside the router, e.g. by using wireshark to confirm that IPIP packets are not arriving when the source is new.

Well, it will be a tricky investigation especially when we have to instruct those that are not so proficient in networking to determine the cause of the problem.

(gateway system, internet router, maybe ISP filters or cgNAT)

Rob