Error: "The following error occurred during backup: DynaZIP error #18."

Damaged data path.

Uninstall Sage 50 and then reinstall it to a new data path. If you are on a network, this needs to take place on the server. The data paths will need to be updated on the workstations and the company data folders moved into the new data path.

Follow How to change the data path for instructions on uninstalling and reinstalling to change the data path, updating data paths on workstations, and moving data folders.