Hello again,
I checked my problem in Hardkernel Forum, but I didn't found a solution.
Therfore I decided do do a clean new install with 4.14 image. After this, everything is fine, no problems shown in dmesg.......
Maybe there were to many updates in history (openmediavault 3 -> 4 -> 5, and also debian 8 -> 9 -> 10).
Hope this helps - don't search and try, make an fresh install and all works fine.
Have a nice weekend!