Since I have seen this failure now in my last project and also got some questions from others in the past, I just wanted to document our root cause as well here. There are already posts out there but the one I found did not really explaining the issue itself my point of view.
In our scenario, we just enabled Virtual Entities for a PoC in one of our lower test environments (not in dev due to some infra topics) and added a lookup (n:1 Relation) to this Virtual Entity. Afterwards we decided to go without the Virtual Entities due to the current limitations and created a Lookup with same name but to a regular Entity. The problem was that the PoC was not reverted before we deployed the final solution in the test environment which caused this (not really helpful) error message "0x80040203 AttributeId is null".
Cheers
