In my case, the only place where TinyMCE does not work is in the forum. I will try to disable plugins and comment on results.

Thank you all for the tips!

Found that the problem was in the Clearfy plugin

Now I will understand what his setting affected the work TinyMCE

Perfomance ->Minify (Html/JS/CSS) -> JavaScript Options -> Aggregate JS-files -must been disable!

Realised my TinyMCE is not working either and a load of content previously created with it now using boxes are not working and are just showing the [box style="1 or 2"]blah blah[/box].

Ok, I discovered the plugin that, in my case, produces this issue: 

https://wordpress.org/plugins/wpo-tweaks/

Besides not loading the TinyMCE bar, it doesn't allow editing posts either. I open a post in the support forum of that plugin, at the moment I deactivated it, and wpForo works perfectly 😉