I didn’t give it a seconds try. When I saw things were made “worse” I thought, let’s just restore the latest backup and get proper advice on how to correct all category tag icons in posts 
I could try again soon, if this is really a proper way to fix it. At the time I knew I was doing something risky, but I thought it would be worth it. My forum isn’t that popular, I could do some experiments (after backing up of course). One downside I also noticed was that some websites are not available anymore. So the HTML previews would never restore. So ideally I only update categories. But I suppose such a HTML rebake is not possible.