One of the mounted volumes got moved to a directory (prob. by an accidental drag-n-drop - need to test the behavior though), so I tried to recreate the folder and mount the volume to its original location. I got "folder does not exist or already a drive path to some other volume". WTH?
Some googling turned up this link http://groups.google.com/group/microsoft.public.windows.server.general/browse_thread/thread/570ac48bce7413ba?pli=1
Essentially you need SYSTEM to have full control on the empty folder to create the junction point, but the created folder inherited the parent, which was configured to not have that - probably an oversight when setting up sharing.