Yes it's indeed the Wifi-6 but a bit stranger than 0 or 1.
After I turned Wifi6 off, the device intstantly connected. After that I tried to turn it back on and rebooted the eReader. Funny thing is, it is still connecting now without any problems even Wifi6 is off.
Maybe someting to do with a not well working Bandsteering if the device could connect to 5Ghz Wifi6?
The reader is only capable of 5 Ghz so this is what makes me a bit confused why this worked.
However, it's running now even Wifi 6 is on.