This one started to work (strange if you did not change other things than logging).

Anyway, this is what I did:
-Uninstalled QSpot
-deleted cache and settings folders
-power off from the phone
-power back on
-installed QSpot
-launched with active internet connection
-marked one playlist syncable in offline mode
-quit QSpot (from upper right, not log out button)
-ended internet connection
-launched QSpot again -> works

Big thanks for your help (and of course for this awesome application!)!