thanks for your reply; but it did Work on IQAudio‘ Buster test image mentioned above (Link) on this Buster-Image, my Pi4 runs without any problems with a fresh compiled gmrender-resurrect version (with all necessary libraries) from github gmrender-resurrect.
I compiled this version exactly in same conditions on latest max2play Pi4-buster version and launched it manually without any connection to the pre-installed version except hardware infrastructure within it’s configurations.
In my opinion there must be a bug in general settings due to the different soundcard and mixer settings, but I cannot find the clue.
If you have a IQAudio DAC+, try my example build and verify please.
Are there any similar bugs known for HifiBerry, etc.?