User Profile Service - Event 6801 - Required attribute 'AccountName' is missing

Client has a SP 2010 farm with UPS configured. Since a while the client seems to have problems with the synchronization. When I look at the evenlog it shows a lot of errors 6801.

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> Microsoft.Office.Server.UserProfiles.InvalidValueException: The property value was invalid.  This could be caused by invalid non-text characters.
   at Microsoft.Office.Server.UserProfiles.UserProfile.Commit()
   at Microsoft.Office.Server.UserProfiles.UserProfile.BulkPropertiesUpdate(Int64 importExportId, Hashtable properties, String strAccountName)
   at Microsoft.Office.Server.UserProfiles.ProfileImportExportService.UpdateWithProfileChangeData(Int64 importExportId, ProfileChangeData[] profileChangeData)--- End of inner exception stack trace ---
   at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Microsoft.Office.Server.WebServiceDirectProxy.WebMethodInfo.Invoke(Object webServiceInstance, Object[] args)
   at Microsoft.Office.Server.WebServiceDirectProxy.Invoke(String methodName, Object[] args)
   at Microsoft.Office.Server.UserProfiles.ManagementAgent.ProfileImportExportDirect.UpdateWithProfileChangeData(Int64 importExportId, ProfileChangeData[] profileChangeData)
   at Microsoft.Office.Server.UserProfiles.ManagementAgent.ProfileImportExportExtension.Microsoft.MetadirectoryServices.IMAExtensibleCallExport.ExportEntry(ModificationType modificationType, String[] changedAttributes, CSEntry csentry)

I checked Miisclient:
MOSS-EXPORT-xxxxxxxxx fails with ma-extension-errors
After I click on a specific error and "validate schema against object" I get the error Required attribute 'AccountName' is missing
I completely removed the UPS.. recreated it, but the errors still appear. 
I really have no clue how to fix this, I hope there are some good suggestions here.