Cant board The Den

I've seen a couple of people with this issue, and they have often managed to resolve it with one of two workarounds (though one requires a PC and console commands:
  1. Dock with the station, then get up from your chair and try the airlock (rather than trying to board using the keypress).
  2. Use the coc StationTheDen command to teleport into The Den station. Be warned that this doesn't fix the airlock issue, so you'll have to commit a crime and allow yourself to be arrested to escape.
It's a frustrating bug, but there are several quests that require you to visit The Den, so either of the above will resolve it until Bethesda or the Community Patch (if you're on PC) have patched it.