Topic: Ubuntu 18.04 with RME 9632: strange distortions first 7 sec playback

I need help, since I am stuck. I switched to Linux 2 years ago after being thoroughly annoyed by Windows 10 (which still is in dual boot however).
I am having trouble with Ubuntu 18.04 and a RME 9632 card (it is card 0, card 1 is the built in sound of the mainboard, which unfortunately cannot be deactivated in Bios but did not give me trouble with Linux and any other sound cards so far). I want to do simple Audio playback (from Streaming source or local file) and route the output to the 2 anolog out (RCA) channels (to my amplifier). The 9632 Card is immediately recognized by Alsa. Not many options in Alsamixer (as I previously read on the web). I started hdspconf, settings seem to be ok, only buffer size settings are missing (compared to Totalaudio). I opened hdspmixer and tried the preconfigured profiles. Profile 2 is somewhat working, but with some strange glitch. Whenever I start playing music (across all channels) reproducably the first 7 seconds are distorted, like sound echoing almost (amplitude oscillating), then it is mostly playing ok, only some occasional cracking, especially in sequences of high dynamic.
I was thinking it may have got something to do with buffer size settings, but which cannot easily be changed neither in Alsa nor in the HDSP tools. I searched the web and found differently configured .asoundrc files with dmix subroutines containing buffer sizes. I tried several, but the oscillating first 7 sec are still occuring (although cracking was reduced).

I then booted up Windows 10 64 bit built 1903, installed the RME driver (which also updated the Bios of the card), rebooted, started Totalmixer. I also found profile 2 to work, chosing the right output in Windows menu (when you click at the loud speaker symbol). And it works, no problems at all, totally smoothly ...

I am really, really frustrated because I love Linux and do not want to switch back to Windows, but I bought the 9632 (second hand) and obviously want to make use of its fantastic sound!
Any suggestions how to solve the issues in Ubuntu?