Did you use the same Add-Devices Rule to enrol the Nokia and Zebra TC51 devices?

What approach (afw#mobicontrol hashtag, QR-code, NFC, ..., etc) did you use to enrol your Nokia and Zebra TC51? 

Hi Raymond,

our Nokia is enrolled via afw#mobicontrol and the zebra with a stagenow profile (scancodes).

Are these devices out of the box?
I had this issue and resolved by resetting them.

Also on wifi there is "x", so you are using internal network? Maybe need to open ports for communication to google servers. You can test this by creating hotspot and seeing if this disappears.

No they are already used and yes it's internal network (same as for the Nokia's so this isn't the problem).

But something strange happened now: i relocated the device and now it's working fine. I'll test again and see what happens.

The AE service account assignment can sometimes be delayed, even if your ports are open to the right servers. I would have guessed that you maybe had different Enrollment rules and you configured the rule for the Nokia devices with the skip the google account creation during enrollment option. It sounds like it may have actually been a timing issue in this case though. 

I was playing arround a bit now, but it only gets solved after relocation of the device.

I don't know why, the only difference is that there are additional profiles which will be installed but nothing regarding permissions.