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You migrated the host OS to Ubuntu? Does ufw happen to be running? RKE2 and host firewalls aren't the best of friends. I believe

sudo ufw disable

is the command to disable it.

Yes, I did. And finally I found the issue, which is: during the migration, I consolidated two networks, making the RKE hosts part of another 'non Kubernetes' network. In conjunction with the loadbalancer advertising virtual ip addresses and propagating those via OSPF to the gateway,...well, there is asymmetric routing now. Which leads to seemingly random packet drop and connection timeouts.