I wonder if some aspect of “syncing” - the enemy of accurate accounting - is in play here?
If you have mobile and web synching enabled then turn that off. Create a new, empty, data file and using that file delete the “cloud” account associated with your live file. Then find the backup that you want to open and copy it using Windows File Explorer, and make the copy into a QDF file by removing the “-backup” that's to the right of the period in backup's file name.
Try again. Quicken will want to “sync” this file to the cloud so don't close until that's done.