Are you stuck outside or inside?
If outside, did you defeated all three of the dragon slayers after Rhode left? A couple people tried sneaking around them, but even with the key the door can not be opened until they are dealt with.
Maybe try heading to the fjords (mountain path to the far south west in Broken Valley) and then return, and see if that resets anything.
If that doesn't help, if you have D2:DC try loading the save in the Developer Mode version, and use the cheat window (F11) unlock function on the door, and see if you get the cutscene conversation that completes the Paper Trail quest when you enter.
If you are inside, the next door isn't opened with a key; read the book on the pedestal for a clue, or simple search the room.
There are 4 vases in the room, one of which can be selected and used like a lever to open the door.