Did you stop the adapter instance and re-start it since you change the creds in vR Ops? When you say "registration" credentials.. there are reg creds used to reg the vCenter extension, then the collector credentials. You need to change the 'collector' credentials, not the 'registration' credentials, to change which account is used to access the vCenter(s) for data collection.
If you find this or any other answer useful please mark the answer as correct or helpful.