O.K. I think it’s the same issue as in Feedback for Foreman 3.11 & Katello 4.13
I have updated, ran foreman-installer, stopped everything again, ran foreman-installer again, rebooted. No change.
Then I downgraded candlepin to 4.4.10 and restart tomcat.service and all of a sudden everything is back to normal, fast and smooth. No more timeouts. Everything is recovering.
Thus, it seems to me as if candlepin got stuck during startup, slowing everything else down, too.
I have got some logs collected, if needed, but revert the vm for now.