This is just too funny, replying to my own messages. I think I found out what the problem was.
So far it has not happened again.
I saw that J!1.5 was redirecting to itemid=66 (which might be different for others and probably is). This was a link on my usermenu "Private Message" that links to the uddeIM inbox.
I had it unpublished, so when it tried to redirect to the unpublished link of course router.php didn't find such a link hence the error