Hello Wim Aerts,
I have checked withe the develpment team for Class Editor and here is what I have received back.
This is generally caused by a custom attribute whose class definition has had a a property removed. Ideally we would like to have a more meaningful message that informs you of the missing property. This will be investigated for inclusion in a future release.
In the interim no issues or problems result from receiving this message.
I hope this helps,
Tony DeRosa
Bentley Plant