Glad you found a fix. That said, the back up is IMHO kind of a waste. It only backs up the catalog. What about your images, presets? A better approach is to dedicate a drive to all these important files (first set preferences to Store Presets with catalog), then clone that to another drive. Back up the whole shooting match. Plus you can take that drive onto any machine that has a copy of LR, Mac or Windows, double click on your catalog and you’ve got everything you need to keep working. Clone back to the older drive, it backs up only the new data and you’re done.
Maybe someday, LR will have a full and useful backup strategy. Ain’t there today.
Maybe the title is misleading, but the Lightroom backup feature always to me said clearly that it is a catalog backup into a chosen folder.
My backup strategy is to use Time Machine on my MacBook Pro (my one and only machine) on two external drives. One drive is a portable 2.5" drive that I always have with me on travels and the other is a 3.5" drive sitting on my desk. Most pictures reside on a 2TB external drive which is on my desk and this is backed up by ChronoSync to 3 other 2TB drives regularly and one of the drives always is in another physical location. I synchronize the Lightroom backups also onto these drives. I use read verify on the ChronoSynch setups.