what i cannot explain: I have not touched the box in many days, yet the problem only manifested itself today. The logical conclusion should be that it is something external. That is why I checked the DNS resolution: If the login process queries a URL a couple of times, but now the DNS server is timing out, this would explain why suddenly everything takes order of magnitude longer. I found a related thread here in the forums where tweaking the /etc/hosts file was advised… it did not work in my case.
https://discourse.mailinabox.email/t/roundcube-now-giving-504-gateway-timeout-after-upgrade/8714/12