Passenger door won't unlock after driving (2008 Passat)

A couple weeks ago, my 2008 Passat developed a really weird problem with
the unlock function on one of the passenger doors.
After driving fast enough (over 10mph) to trigger the auto-lock, one of the
passenger doors won't unlock with the fob or internal unlock buttons (either on
the driver's side door or on the passenger door itself). The only way to unlock
the door is by pulling on the internal handle, then you can open it with the
internal or external handle--otherwise the "locked" light on the door remains lit,
and it won't open.
The weirdest part is that this problem only occurs with the auto-locking
function, and only on one door. If I've already used the internal handle to
unlock it, then I can (re)lock the door, and it will unlock again with the fob or
either of the internal buttons (driver's door or passenger door itself).
Anyone have any ideas about what could be causing this?