This isn't a dig at W3TC or wpForo, just an "aha!" moment that just because these two plugins are very tightly integrated with Wordpress doesn't mean that Wordpress tells one about the other's actions.

Adding (for us) /community* to W3TC's Never cache the following pages: list seems to have solved this for us. :)  

What cache plugin do you suggest?

I need something that will work with cloudflare & my CDN?

If you need CDN then WP Super Cache is the best in the free zone.

If you do not need CDN then Comet Cache Lite is by far the best out there. If you can pay 40$, go for Comet Cache paid version that supports CDN.

Here are information how to exclude it:

https://www.webnots.com/exclude-pages-and-directories-from-caching-in-w3-total-cache/

I need something that will work with cloudflare & my CDN?

I can't figured out how to exclude all the forums in w3TC.

Still using any caching MUST exclude all of wpForo pages.

How should I configure W3 TC to work with WPFORO? 

How do I exclude WPFORO Pages?

Yes W3TC no reason to use that in 2018. There are much better, safer, simpler and modern cache plugins. W3TC is an Open Invitation for Trouble.

But of cource everyone has his opinion.

Since wpForo has his own cache and since wpForo is NOT compatible with any other cache plugins, you will have to exclude the Page with shortcode. I dunno how it is done in W3TC.

@louie,

Please delete all caches, then deactivate those for a while. Then make sure "HTML Cache" is also disabled in Dashboard > Forums > Settings > Features Tab.

Then press Ctrl+F5 and test it again. Also please leave some URL to allow test it too.

I deleted the cache.
"HTML Cache" was already disabled.

Everything is really slow without my CDN (which was using W3 Total Cache).

What do you mean by "leave some URL to allow test it too."

I only said that using all that, it is impossible to test. Use what you like, after you have a stable system.

LOL, but you said "that crap W3 TC"

Please delete all caches, then deactivate those for a while. Then make sure "HTML Cache" is also disabled in Dashboard > Forums > Settings > Features Tab.

Then press Ctrl+F5 and test it again. Also please leave some URL to allow test it too.

Never said wpforo doesn't work with something. Still using any caching MUST exclude all of wpForo pages.

Also using DEBUG.LOG could provide you info sometimes about the problem.

Cloudflare protects me from spam bots and other nonsense. 

This software won't work if you use cloudflare?