Turns out I didn't actually fix the problem since it popped up again a day later.
On the plus side: I figured out my SMTP problem from earlier. My server host blocks port 25 outgoing for the first two weeks that a VPS is running, regardless of the fact that I've been a paying customer for more than 2 years now.
As of yesterday morning, mail is now sent properly from the server, this issue should really not be popping up any more.