Last tested on Version 18.104.22.168 beta from my last reported occurrence.
I still owe you a test where I build a new server with the latest and try a backup generated from a 3.6 install. I'm expecting to have this tested today as I build a new installs for 2 users I'm helping.
Here is what I usually do:
After installing the deb, I then log into the web ui, complete the initial setup wizard. I then add the premier key, then go to plugins, search for backup and restore and install. Then I force a restart using the web ui. Then after restart, I go to plugins, open the plugins, open backup and restore. Set the backup location to the location on disk, it lists the backup(s) I have saved to the disk (with permissions emby:emby), then I click on the listed backup, and the backup information screen loads but has that hanging effect. If I switch browsers, do a restart, etc, doesn't help -- still hangs. But if I give myself a "time out break" and go get a coffee, etc and revisit it some time later, the backup information will load.