After the v1.5.3 my moderator gets error 500 on her browser time to time. She can sometimes open the topic but sometimes not. I mean the same topic. Also while sending a message. She can succesfully write the post but she can't see it, because again 500. I didn't understand. There's nothing wrong on my side or other members. We even tried at the same time but I can open topics while she can't. So I think it's not server related. There must be something wrong with her wpforo account.
Can it be? I'm out of ideas.
from my experience, error 500 is as result of maybe multiple sessions when using NAT protocol.
Reset modem/router, clear cookies from browser and you'll be good for few more days.
I have the same issue with one moderator-account. The problem is associated with the account and not with the browser/router etc. We have done some tests with multiple devices.
Web server error log:
Just follow the standard WP debug procedure guys. Debug.log, default theme, disable plugins one by one. etc etc.
Also what is your PHP version and your php memory allowance.