OAuth authentication failed since update to 7.5 SP16

Hi,

we have a RFC-to-REST scenario with OAuth to fetch a token in REST receiver channel that worked without any problems. After upgrade to 7.5. SP16 we receive the following error:

[EXCEPTION]

Ausnahme aufgetreten beim Adapter-Framework: HTTP OAUTH 2.0 RESOURCE OWNER PASSWORD CREDENTIALS GRANT call to https://<URL>:443/token not successful. Error while processing Authorization request!

com.sap.aii.adapter.rest.ejb.common.exception.HttpCallException: HTTP OAUTH 2.0 RESOURCE OWNER PASSWORD CREDENTIALS GRANT call to https://<URL>:443/token not successful. Error while obtaining authorization code - response code: 400
response:
{"error":"invalid_clientId","error_description":"Client 'xyz' is not registered in the system."}

Testing with Postman, the user is still available and I could fetch a token there (what I could before upgrade in PO as well).

Are there any changes in the OAuth process with the last updates?

Regards

Chris