Infusionsoft and Converkit mean nothing to me but this may possibly be the cause of the 502 error. But let's talk about the access denied one first.
It is possible that there is a problem with the permissions data - rare but it can happen of course. So - go to the forum admin > toolbox > housekeeping and click on the task to 'Reset Auths Cache'. See if that resolves the issue.
As to the 502 - there is a reasonably good explanation here https://www.lifewire.com/502-b.....ed-2622939 which discusses likely causes and things to try but the bottom line is that your web server is trying to connect to a third party server and is failing to do so. If this only happens on the forum page then there must be something on a third party server being requested. Could be in the post content itself although if intermittent that would suggest something like a third party server working above its bandwidth limit for example. Your host support may be able to look at the server logs and pinpoint the cause.
I do note you are using cloudflare. That can be very tricky to set up and get working fluently so it might stem from that...