Dear @Hellothere,
Welcome back at Wefugees and thank you for reaching out to us again.
First of all, congratulations to your new job - I hope you will like it!
From your explanations I can not really tell why they want you to be present that day. I am also not too familiar with these kind of procedures. I doubt that this requirement has anything to do with the re-registration regulations (Ummeldung). I think it would be the best to discuss your issue with whoever told you to be present that day and explain them that you actually have to work.
Sorry that I can't say much more about this matter - maybe other community members can add to my answer.
All the best,
Meike