Sometimes, although probably not the case as some that have more experience with DL (I am avoiding DL until I cannot avoid it....geez I used live in the NE and had to fly that airline all the time: it sucks, well to be technically correct and it is my own opinion, it became a crappy airline after deregulation--it used to be a premier airline in the pre-dereg days) and they seem to have your problem figured out. HOWEVER, sometimes one gets "lucky" and you can not get an assigned seat or an online check-in either because you have "privilege" of a careful TSA check.
This usually only happens when you try printing tickets...you get a message telling you that you cannot have your ticket printed, but it happens when trying to pick seats as well. I have had both happen to me.
Andy