The reason your forum comes back online after Discourse-Doctor is because it’s restarting the existing, pre-rebuild version of the site. So, you’ll be online, but unfortunately you’ll also be back at square one.
If you have a recent back-up, you may find it quicker to spin up a new droplet with a fresh instance and restore your site there?
I’ve searched the site for similar errors, as I’m afraid it’s not one I’ve got any personal experience with, but I did not understand the answers enough to be able to advise on them. 
@pfaffman is normally good at these. Would something like this work, or is it a total red herring?