Thank you all for your help

Finally i found the issue by testing Recalbox with a Mini PC instead of the RPI4.
It first booted without any videos, too.
Then I went according to the exclusion principle because the video was suddenly played after only the USB stick with the Recalbox image was connected to the mini PC 
Then i connected everything back to the RPI4. No Video...
The solution so far:
If everything is connected except the both Dragonrise USB Encodern then the bootvideo plays...on pc and RPI4.
The problem is caused by the encoders !!!
As far as I know, boot videos can be skipped with 9.0. Can it be that some function is activated by default with this encoder type so that the boot video is thereby skipped?
What can i do now...i have no idea but good to know who is responsible for the problem....