I want to update my case for other folks with the same problem like me.
I opened a ticket at discord. There the support is pretty active. The advice was to try to onboard the miner with different apps, like MNTD., bobber app and Hotspotty. With all, the registration process ended succesfully, but the miner was not linked to my wallet.
You can also use the bobber app for diagnostics or the Hotspot Utility.
If you own a device with a CalChip like me, you can check this site. You see there the Remaining Onboard credits. If there are no credits, you can’t onboard your device. You just have to wait, untill a credit if aviable again.
I onboarded my Rak with the bobber app (Bobcat miner 300).