Problems on OSX missing file errors - unable to open on firefox "problem loading page"

I am not well versed in computers so i need simple solutions. I am running the latest beta version of Duplicati on OSX version 10.13.6 on a mac pro. I can’t open duplicati anymore on firefox 65.0. At first it worked fine. I setup a backup to a local backup disc (not in the cloud). It ran well. Loved the ease of use setup custom backups. It did everything I wanted. So i was very disappointed when the second backup “failed with missing files on remote server error”. Not sure of the wording because I can’t open Duplicati. I did the repair as suggested but the backup failed with either the same error or just a simple error message. I have tried many times to run an automated backup with no success. I can’t be solving issues every time a backup runs. I am not very computer savvy so I need help that I can understand. Hope you can help. Thanks.