I understood the problem. I followed the advice of a post on WPforo to size the sidebar differently. The instructions I have inserted in the CSS are the following:

# wpforo-wrap .wpforo-content {width: 73%; }
# wpforo-wrap .wpforo-right-sidebar {width: 24%;}

Now I have removed the instructions and everything seems to work.
Strange though ..

My theme is absolutely "Mobile-Friendly" and has never given me problems.

Thanks to everyone, everything works now.