Restore button does nothing

I have same problem as person in this thread: https://wordpress.org/support/topic/restore-site-does-not-work/

I wanted to restore an old site to a new install, I uploaded the backup file, but when I click the ‘Restore’ button, a loading symbol flashes for half a second and nothing happens. If I try to get Restore Log, I can’t because there is no option (I’m guessing because the restore doesn’t even get a chance to load). This is brand new WordPress install, with default theme, and no extra plugins except Backup Guard.

The only hint of what’s wrong is an error in the console:
https://wordpress-74373-765429.cloudwaysapps.com/wp-admin/admin-ajax.php?param=sg_backup_opt(full)_etonhouse1&action=backup_guard_modalManualRestore
Failed to load resource: the server responded with a status of 500 ()

Any help is appreciated.