DEVICE-0202 Error while adding rSeries as a provider in CM

Hi all,

we just got out rSeries Hardware and I am now trying to add it as a provider to out Central Manager instance.
Unfortunately I am always getting the following error Code when trying to connect it to CM.

DEVICE-0202: BIG-IP Next instance internal server error: Certificate validation failed with error: provider c399-apb20-tl.net.muenchen.de:8888: DEVICE-0202: BIG-IP Next instance internal server error: SSL certificate is unusable: BIG-IP Next c399-apb20-tl.net.muenchen.de has an unknown or unusable device certificate; explicit trust is not possible. Error: EOF. . 

For creating the Device Cert on the rSeries Platform I used to following KB-Article which is for the DEVICE00060 Error: https://my.f5.com/manage/s/article/K000139300
- I issued a self signed certifcate with DNS and IP as SAN

Since the error didn't change I tried adding a few other things:
- Adding the self signed cert to CentralManager via Applications>Certificates & Keys>import 
- Creating a crt signed by out Internal CA (no IP in SAN cause the CA doesn't allow that), applying it on the rSeries  and uploading the chain to cm
- Testing if the rSeries really uses the applied certs on port 8888 each time with openssl s_client connect which it always does

But all that didn't change the error message at all. Any ideas?

Thank you in advance