I updated the site’s software today to the latest version, since there were serious updates available.
A BIG MISTAKE!!!
It took a few hours of my time, because the fucking CATEGORIES tag was gone. And it was impossible to make a new category.
Basically what happened is that the whole database structure changed (some tables) and it was not up to date.
After searching for hours I found out about this, as their forum and support pages aren’t that helpful at all.
I thus made a backup of the database, and the posts in WordPress extension, just to be safe, and reinstalled the whole database.
After that imported the whole thing, and made new categories and edited the posts, as that was the only option left to do.
Now everything is back up and running!