I constantly get a page showing a post is offline. The main forum page work, but lots of individual posts won't open.
Helpful if you could tell us - better yet, post a "screenshot" - of the exact error message.
Sometimes it is a network enhancer. Sometimes it can be a garbled/expired pointer that confuses the PM database call.
Ex: I usually see this at times when the end-to-end link has high packet-loss percentages and/or high link congestion & latency somewhere in its routing. East coast USA host server to end-Luser in Norway, that is more likely than a near-as-dammit "local" link, here.