I’ve encountered the same problem. I noticed that all my backup files since Oct 8 had the same issue. An underscore was mysteriously added to some .css and some .js files. In my case, the following files were affected:
/theme/theme_name/css/bootstrap.min.css -> file name became bootstrap.min_.css
/theme/theme_name/css/fontawesome-all.css -> OK
/theme/theme_name/js/bootstrap.min.js -> file name became bootstrap.min_.js
/theme/theme_name/js/popper.min_.js -> file name became popper.min_.js
Not sure why the fontawesome-all.css was intact when there other files were affected. I hope this additional underscore was all this issue was about. It will be greatly appreciated if you can advise if there are other issues as I rely on BackWPup on a daily basis and if the issue is more extensive (i.e. affecting database, etc) I will have to stop using it until everything is fixed.
Many thanks!