Found the cause.
Shield plugin with enabled:
“Block PHP Code Includes
This will block any data that appears to try and include PHP files.
Will probably block saving within the Plugin/Theme file editors.”
did that. Disabling that option and removing and reinstalling your plugin worked great.
Sorry for the trouble, i couldn’t imagine that.
Did more test on this and it seems that The Shield is not the problem. I re-enabled those options, removed the forum plugin and re-installed it.
This time all is well. Strange.
I dunno what happened in my first attempt, just wanted not to blaim The Shield plugin that has server me well.
Hi MassimoD,
it seems wpForo Navigation menu is not attached to wpForo Menu location in Dashboard > Appearance > Menus admin page. Just select “wpForo Navigation” and check the wpForo menu location:

Also make sure the menu bar is enabled in Dashboard > Forums > Settings > features admin page.
Thanks for the reply.
I have not touched those, ever. Neither in my 1st failed one, nor the second that works.
Nice to know where to search.
Many thanks