I just wanted to say thank you and state, that this step was the most important.
Adding the 915 option via modprobe.d (like Glutao suggested) was always purged after reboot, but adding the line directly via this entry made everything work (Unraid 7 beta 4)
Now I can finalize the setup of my Austor 5402T with Unraid and Jellyfin.
BTW: You can enable MPEG2 acceleration too, if you still have some DVDs.