Hi Ronny Kaptein,
Thank you for posting in the Microsoft Community Forums.
Are your two DCs in the same domain?
After you perform the manual replication, the contents of the two domain controls will not be replicated right?
You can check what exactly is being reported by looking at the corresponding error events in the Event Viewer.
Regarding the "Lingering Object" you mentioned, is it possible that you have a domain control that is not used anymore and is still in the domain?
Here's an article that guides you on how to make changes to the Lingering Object error:
lingering objects in AD forest - Windows Server | Microsoft Learn
If you're experiencing other issues, you can post a screenshot of the specific error or a copy of the text in a follow-up reply.
Best regards
Neuvi Jiang