And thanks to Mr Richard Lantaville for reproducing it today in his thread, I managed to find at least 3 messages so far that caused this problem.
I will not reveal what it is yet because it's not fixed, but I can assure you it's fixed and it won't happen again, or at least, it will not come back after 1 min after I resetted the server.
The problem was a message posted by Huntsman in the Homer Odissey thread, it's now fixed and well, it's working.
I'm currently processing all the messages on NOOL to find other problems like it, so the system might be slow, and forums might show empty messages until it's done.
Sorry for the problem!