EFilerError is a rare issue at the startup of coDiagnostiX.
It usually occurs with versions older than 8.5.0 and can be solved in a few minutes by deleting temporary configuration files for the Online Update.

1. Navigate to the coDiagnostiX configuration folder (How to)
2. Close coDiagnostiX if running
3. Check if the Online Updater is running and end the process using Task Manager (OnlineUpdater.exe)
4. Create a backup of and delete all XML files in the configuration folder which names begin with OnlineUpdateData_ or VersionInfo_.
   Examples are shown below:
   
5. Transfer the Backup of the files to Dental Wings for further diagnosis (coDiagnostiX.support@dental-wings.com)
6. Start coDiagnostiX and check again

Please note:
Be careful not to delete any folders or INI files in the process.