Activation failed. Undefined error when add a iPhone in Sophos Mobile Device Management

Dear Sir or Madams,

when I try to add a iOS (iPhone7) device in our Sophos MDM I get on the SMC App following error: "Activation failed. Undefined error" when trying to activate device" When I view the SMC APP logs I get a: getStatusUrlList: no 'SmcAppUrl' item in keychain, error -25300.

I think the iPhone doesn't trust our Root CA.

I already tried following steps:

- imported the certificate with the whole chain in the iPhone manually

- try to assign the client certificate and the root CA certificate over the Sophos MDM policies. 

If I now go open the Webserver of the Sophos MDM, I get a trusted site but I still cannot add the device in our MDM. 

Anyone an idea or a workaround how can I add the iPhone in our MDM?

I appreciate your help!

Jonas