Thanks Robert for the reply. So if i get this right, no topics are cached as html static pages, only the forums home (the page we put the shortcode in...

All your sites are just excellent. I envie you. May i ask what plugin you use for cookies/privacy ?

We just released bug-fix 1.5.4 wpForo version. You mean 1.5.5 ?

Well that plugin has the Fremius tracker option(ally), whatever that means for each of us. I would suggest ARI fancy lightbox.

They look totally equal to me. Screenshot.png

Works great for me. Could be your theme ?

Who knows ? Enable standard WP Debug to a log and check for fatal or other errors. Registration and emails are handled by WP.

Or just the hosting tmp path is wrong/not permited, because sessions are stored in there ?

I can only imagine a plugin (that was auto-updated ?) creating that problem.

Well, wpForo-Robert will shed some light in this. Personally i think it is not a wpForo issue, still ...

I have no idea what wpForo debug records (or not). I'm talking about standard WP, Enable Debug to a LOG (in wp-content).

Is your debug log clear of fatal errors ? Blank pages are sometimes "white pages of death" in WP.

Similar to what exactly ? W3TC does a lot, even db caching. I would say try WP Super Cache (Automaticc's plugin) or Comet Cache Lite.