When I click the preview button for any lightning component in my sandbox I get a page which says
We are down for maintenance.
Sorry for the inconvenience. We'll be back shortly.
This also happens when I click "New" on the Lightning App Builder.
There is no error in my log files. It does not even generate a log file. It is almost as if my .lightning.force.com site is messed up somehow.
I don't think there is anything wrong with our components.
I can view the Lightning Components on our community site. And I can create a new blank application which should just show a blank screen when I click preview, and I still get the error.
This is a full sandbox, with lots of development work in it which has not yet been deployed. A few months ago Salesforce restored this sandbox from a backup for us as well.