Problem solved.
For reasons I don’t understand, access to the site stopped working after about 24 hours after I added a TXT record to DNS.
I tried adding an A record with the server IP in forum.domain.com format, and after a while, it started working again. Previously, redirecting to a subdomain with *.domain.com worked.
Admittedly, this is a mystery to me, but the main thing is that it works now 