Something interesting just happened @fitzy!!
I restarted docker on the server… and the forum came back BUT then one of the other ones stopped working!! So I restarted it again and went to this forum first, it failed, then went to the other two and they’re working (they’re more important than this one). Seems very odd but at least we have more clues to work with!
Has something changed in docker_manager that would allow only two discourse containers to work at any one time?
No such change has been done, and since containers are isolation tools first, they are unaware from each other.
I’m afraid your custom setup is too far from our standard install install, and as such not something we can provide personalized support here. It’s also off-topic for the PostgreSQL 13 upgrade. Maybe fetch some help in the marketplace?
I’ve just upgraded Docker and I’m getting the same thing Michael - and it appears that whichever forum is visited first is the one that breaks.
It’s this upgrade that appears to have caused the issue tho Rafael 
it had been working fine for several years previously with possibly hundreds of updates without issue.
I’ve always asked for your guidance in setting up Discourse so I am hopefully following the Discourse way for such a set up (I don’t think there’s anything particularly unusual about this set-up either; running more than one site/forum on a server).
This really sounds to me like something is going on at the host/server level, and not within the app containers. If the site works at all (e.g. you can use the Rails console) then the upgrade to PostgreSQL 13 isn’t the issue.
I was thinking perhaps there might have been something else that was a part of this update that caused the issue - but in fairness I hadn’t upgraded the forums for a while (as I was holding off) so it could be related to one of the other updates.
Shall we split the non-PG posts here into a dedicated Topic to see if we can get to the bottom of it? I’m sure it could prove useful for others too (the guide I wrote for running this kind of set-up is by far my most popular thread here - so it certainly appears that many others are interested in running Discourse on a forum with other sites).
This is fixed now… for some reason the firewall started blocking Docker traffic! 
Thanks for all your help and perseverance @fitzy (and everyone else).
EDIT: On further investigation the server started blocking this container’s IP because of failed pop3 logins. This particular forum is in read-only mode and when it was put into that I removed the email account from the server, but also thought I had switched off Poll via POP3 for email replies in the Discourse settings as well - but it was showing as still on so I either didn’t do that, or I did and it may have got switched back on when/with one of the updates.