I switched to WordPress twentyseventeen theme and the problem is gone. But with the Hueman theme the problem exists. Inside the CSS of Hueman I ...

Inside the forum dashboard, I clicked to the [delete all caches] button. I don't have a caching plugin for WordPress. After this, I still see inside...

Sorry, my fault. The problem is inside the "From:" headline of the email. There is "Tubby\'s forum" instead of "Tubby's forum".

Yeh, it's very easy to say: This is feature, not a bug. At least it's lesser work, than to change it...

... but this does not delete the forum posts, only the WordPress posts and WordPress comments, I read the description well. Did you try, what happen...