Hello Jean-François
This is more than likely related to your import profile settings, have you tried removing all the user GUID's from the import profile so that it grabs them again to see if you still get the error? It is possible that it is not actually checking AD for the user and just using the import profile to find the GUID. Also when you create a new profile and select only the user you want to import does it generate an import profile with only that users guid? What is the value of their GUID?