In the default Joomla log directory, you will find the file "mms_synchro.log.php". In this file, every transaction is logged. The correctly added, updated and deleted members with their member id's. But also the errors that occured during the API or in the process before the sync.
The order in this file is chronological: new errors are added at the end.
Backend error log
In the backend of Joomla the errors are shown that were reported when the API was activated. The errors also come with feedback from the API.
The order in this representation is reverse-chronological: new errors are added at the top.
When a field with the type E-mail or Iban has a wrong syntax, that value won't be synchronized. In the logfile, you will find the member id what values were refused.