The MyContact Entity was only for testing and is deleted.
At the moment i use the standard contact entity.
But i tested with diffrent entities and flows.
But as mentioned in my other post the workorund using the PrimaryId field as the Record Id seems to work.
Usimg the ReordID filed as Record Id results still in the erros.
I will setup an sync flow for one of our SQL Tables.
I will the post further feedback.