Weird behavior of Dashboard, must be core files

I contacted my host service again. They just gave me the same line to insert into my .htaccess file and I told them I already tried it and it didn’t work.

I then showed them my .htaccess file and they deleted the whole part that concerned their server caching. Now server caching is completely off and everything works again.

Still not sure why this previously never caused issues. In the end, it had nothing to do with WordPress.

I hope this answer will help people who run into similar problems.