Nah that ...backup.log is just a backup of the log file (basically the output log text). There is a backup option for assets, but those would end up in your /Saved folder, and you’d said you deleted that folder so that won’t work.
If there were any version incompatibility errors reported, I’d suggest disablings.EnforcePackageCompatibleVersionCheck and trying to open the assets, but you’re saying you can’t even see the assets…
Are you sure you didn’t just filter out the content browser, hiding the assets? 
Like this (where only Animation Blueprints are shown, and my folder has none, but it has other assets which are now hidden):
Or maybe one of these options?
If those .uasset files are indeed within the project root on the disk, UE should try to load them and report if something goes wrong… so maybe they are there but just hidden? If you start your game, do you get any errors indicating that stuff is missing?