As I solved this issue I just wanted to provide the solution to other users.
In the current DT register version 2.6.5 there is a small bug in the de-DE.com_dtregister.ini
Line 76:
Wrong:
DT_Location=Veranstaltungsort
you need to correct to
DT_LOCATION=Veranstaltungsort
otherwise in the Eventlist in the frontend you will see DT_LOCATION (wrong) instead of Veranstaltungsort (correct).