Suddenly, no one can post in the forum in every topic. Not even the administrator!
Few hours ago everything worked normally. I didn’t do any update or any other change.
Also, I disabled every plugin but the problem persists.
I appear to be having no problem adding a new topic AND a new post to that topic.
You will want to delete my topic (test) of course.
If you can not post I would suggest you start by closing your browser down completely and then reloading it.
You just added a topic and it accepted it?
But nobody could write a reply. I got a lot of complaints from my members 
The error happened with both the desktop and mobile version of the site.
Correct.You can see it in your forum and, as I suggested, you should probably delete it.
To be honest I have no idea. I saw nothing wrong There were no php errors and the only console errors are missing images causing 404s which are not enough to stop the operations.