NodeBB's cache buster is based on its current git hash (so if you've upgraded it will try to send you the new CSS and JS, which works well, but it remains the same if switching themes, which results in your issue) I suppose the dev team hasn't run into this because we usually run with cache disabled while testing (and because Persona is so awesome so why switch? Kidding) https://github.com/NodeBB/NodeBB/issues/3573

Bloody hell! I took out the recent replies option on the category causing trouble. I changed it from 2 to 0 and that solved the issue. I was able to activate the category again and everything is back to normal. I'm guessing the poll was bothering the preview on the homepage.

Tomorrow i can test it at school, at home it says it works.

Done it, thanks again for your help @planner .