Thanks, IgorG. But if any of us was infected with something that modified hosts file, then logout/login or clearing cookies would not help, and yet it does. So this is either nginx (or other backend servers) misconfiguration or forum software issue.
Just for the record, my hosts file is modified, but by me. And it does not contain suspicious entries.
Next time I encounter this issue I'll try to debug a bit more. Currently I don't have time to reproduce it.