[@janiszewski99 Participant 6 years, 10 months ago]

Problem solved!

I have disabled the plugins:

WP Fastest Cache and
WPBakery Page Builder

It helped —> I could already add a new forum.

The strange thing is that when I turned the plugins back on, the site still worked correctly 🙂
Maybe the problem was on the cache side …

Thanks for pointing me to the solution 🙂