.DE - Update of the nameserver not possible (Error Code 549 Description: Command failed; 63200020603 This contact is administered by another provider [DENIC-1000100-OPS])
The nameserver of the .DE domain cannot be changed after transfer back from the DENIC transit.
The .DE registry is mentioned as Tech- and Billing-c if the domain was listed in the DENIC transit. You have to re-place these contact handles with your handles.
Afterwards you are able to execute further updates like nameservers update. You can read more about .DE in our wiki or see the command in our API manual: MODIFYDOMAIN