While working on an Azure ADConnect deployment we had a few users with export errors. The users seemed to sync into the cloud so I was not fully aware of what the export error actually affected but… it was still there..
Anyway I found the following article that explains why you get the error and how to fix it. Its basically down to the security settings of the users profile in AD. Basically, enabling the inheritance solved the issue and the ADConnect was able to export these identities.
User > Properties > Security > Advanced > Enable Inheritance