I have been using WordPress for a while now and today I noticed something going wrong on my site. When I click a category, archive or tag the appropriate page opens but displays no results. I've also noticed my tagcloud is missing some often used tags. This is somehow related since both problems emerged at the same time.
Links to individual posts or pages work just fine.
I'm using "post name" from the permalinks settings as my default post title.
The site in question: