marx_w00t
Okay got my problem solved.
My forum is at /var/www/html/forum
I just renamed the folder to forumold (it doesn't really matter how you rename it). Then I created a new database forum4 + completely reinstalled flarum in the newly created forum folder. Rights etc all assigned. Installation carried out without problems on the new database.
Then I gradually exported/copied the most important tables such as posts or users from the forum where I had this problem into the new forum4 database. At first, copying the individual tables post, tags and discussions was not easy because I got this "Foreign key constraint is incorrectly formed" ERROR CODE in phpmyadmin. I then simply clicked on further details in the error code at phpmyadmin and there was always a table in it which probably triggered this error. Deleted them and copied the table again. And then the same thing over and over again until all the important tables such as posts, tags and discussions were there again.
Then simply reinstalled all the extensions that I had without any problems.
I don't really know much about databases, but it somehow worked.
Now everything works again without any problems.
