Ok, just an update to this. We spent weeks to find such problem with HTML Cache. I'm sorry but we couldn't find it. This has some specific thing which only exists on your server. I'm really sorry but this issue cannot be solved because this is not a clean wpForo issue, this is a result of wpForo affection by other plugins or by server built-in caching solutions. If you can reproduce this issue on a simple servers (without built in caches) with a fresh WP/wpForo installation we'll continue investigation. Just contact us and send the clean (not the existing) WP admin login details to support[at]gvectors.com.

I'n sorry Robert but what you actually ask can't be done, and you already know it.

You ask me to find an other web server (?) and work on it, set it up, make accounts, whatever.

I have NO other caching on the webserver (Litespeed) and there is no conflict of any kind since i already have tried to disable all plugins.

For me all those problems i report (not only this one) exist to other users also, they just do not know it. Hey, most people don't even know what WP debug is.

I do understand that most people here have simple and easy to solve problems like css conflicts, still some of us go deeper than that. 

Anyway, thanks. I do understand your position since i used to work for 10+ years for a web hosting support company in my country. Still a problem is a problem.