Hey Micha;
I'm still trying to help.
Have you disconnected the HDR from the d8b and seeing if the HDR works on it's own? Try disconnecting everything including keyboard, mouse and monitor. See if the HDR will record something using only the front controls on the HDR not connected to anything. Hopefully you can get a signal into the HDR via your I/O's.
If it works, add the Monitor, Keyboard and Mouse back into the mix and see if all is well. Post back with what you find.
If it doesn't then perhaps the HDR is at fault.
Mackie may or may not be able, or willing, to help. Good luck with them.
Failing help from them, Eric Russ has, or had, a tech for his d8b and I believe he know the HDR fairly well.
You could also try Mike Rivers. Just google his name. You should find him. He was part of the the team who wrote the manual on the HDR and has published a book on the HDR. He is the MAN.
Good luck and cheers.
Bruce