What means the error message: ”MAPI-Error: MAPI_E_CORRUPT_STORE”?

It seems that your pst file is corrupt, so SYNCING.NET cannot get access to it. You should exit SYNCING.NET, check and repair your Outlook profile(s) and pst file(s) using scanpst and start SYNCING.NET again. If this won't solve the issue you have to delete the SYNCING.NET "Outlook Group" and re-create it.