I noticed some problems with my site so I decided to do a fresh reinstall of WordPress today. I backed up my posts and plugins and deleted everything, and reinstalled everything back again.
Everything works fine, in terms of theme, posts etc. But what doesn't work are the permalinks. I had my custom permalink structure "/%postname%/" and everything worked fine. Now that my posts were backed up AND post slugs, everything has gone wrong.
I've tried to update the structure multiple times, even tried using plug-ins to do this for me. But it just won't work.
I've also tried to use the default until I could get my custom one fixed. But I can't even change that option anymore!