Hi,
thank you for the hint, which got it working for me. However it was the other way around:
if option “Wechsel der Zahlungsart” is enabled, which is the default option, then redirect doesn’t work (endless loop back to payment page)
if option “Wechsel der Zahlungsart” is disabled, then redirect starts working normally, so the user is redirected back to our web site.
Thank you for this hint, for us it’s an acceptable workaround, so I hope you can fix this bug!