Ok, something strange has happened to my WordPress install. I can’t assign categories to posts anymore. The categories still exist, so they’re still useful for older articles, but not for new ones.
I’ve noticed a few posts around with other people having similar issues, and I’ve tried a few of the easier (quicker) suggested fixes, but no luck yet.
Hopefully I can find time soon to really investigate this properly. Until then, just be aware of this issue.
