by Tim Schroeder.
I just ran into the same issue. I debugged the process with xDebug, set a breakpoint in backup/controller/restore_controller.class.php#L455 and looked at
$this->precheck->errors to find that it contained this message:The backup file contains more than one top-level question categories for context 3304688.
It would be nice if this message was printed directly to the console so we don't have to dig for it.