Hey, Willem- since Ubuntu did some sort of update to XOrg, I've been getting this, usually after a suspend/resume cycle:
$ disper --single
deletion of dangling metamode 50 failed
unrecognised scaling value for DFP-0 from nvidia-settings: 131073
... and it doesn't disconnect the secondary display (but moves all the windows off of it to the primary). If I run "disper --single" again, the X server must crash hard, as I get returned to the GDM login screen.
I first thought it might be related to the 195.36.31 NVidia driver, 'cause I saw it there first, but after trying 195.36.24 and .31, and 256.29 and .35 and seeing it across all versions, it's obvious this is due to Ubuntu's update.
If you need more info, let me know.