Invalid file - not a PKZip file

The error message "Archive Process spooler entry error: An exception was raised on the server: AddMailTransactionFile Exception: ... Invalid file - not a PKZip file" is the result of a corrupt archive transaction (mail).

This seldomly happens and is mainly caused by system failure (PowerOff / Reset / Powerfailure).

This transaction cannot be fixed.
It only can be deleted via Admin Webinterface -> Maildepot -> Archive Spooler -> right click -> delete.