Thresher Ultimate Xbox One X - random noise / clicking noise

Greetings from Germany!
Have a two weeks old Thresher Ultimate for Xbox One and have set it up acc. to the manual. It worked fine so far.

Just yesterday the headset's volume got extremely low and I permanently hear a random noise in the background, plus some clicking noise from time to time. If I push the Dolby button on the station, it permanently sounds like sitting in a tin bucket.
As I said, this negative effect came out of a sudden. It worked before and I didn't change any settings!
Strange: If I switch the station to PC mode (whilst still connected with my Xbox), sound is excellent again. No clicking, no noise, but it is extremely loud and whilst in PC mode, you cannot turn down the volume....

My Settings on Xbox One X (translated from German):
HDMI Audio: Headset
Optical Audio: Bitstream
Bitstream: Dolby

On the right side "Headset Audio" appears then:
Headset: Windows Sonic for Headsets
HDMI or optical audio Headset: I have checked the box (yes)

Have tried differend settings as well, but without any positive result.

Well, this is my main problem and hope to find a solution here, but I have two more questions ;-)

Surprisingly the headsets works fine again (still on Xbox!) as soon as I remove the optical cable from the base station! So it is just linked to the Xbox by USB cable!
Question No.1: What do I need the optical cable for? Does it provide a better quality (7.1)? What are the benefits of the optical cable?

Question No.2: Is it possible the a.m. problem is a result of a broken/damaged optical cable? I will go and buy a new one to checkt this, unless you guys stop me and say "No need to buy a new cable. I had the same issue. Just do this and that and it will work fine again....."

If I connect the headset to my PC it works fine. No issues.

Thanks in advance!
Dennis