A number of people have noticed and commented on Moodle.org (see https://moodle.org/mod/forum/discuss.php?d=353722#p1428201) that when restoring courses, their Moodle's cannot be used, essentially they are locked out of their current browser session until the restore finishes.
It seems to be a known problem (as suggested by Tim Hunt in the above moodle.org post.)
This Tracker item suggests that this should be researched, and if the session is really being locked during course restores, that this should be changed.
I believe that this problem has existed for awhile, but it certainly exists in Moodle 3.3.2.
As I searched Tracker, I did not find duplication of this bug, but I did find a few others that might be related.