I rarely wake up during the night but last night I did and had a couple of emails about a couple of Mastodon instances having trouble federating.
This was at 5am and after 14 hours (it's now 7pm) I found a solution to the problem, just removing 2 's' from the Mastodon code: https://github.com/tootsuite/mastodon/pull/12812 
@noellabo It's possible that all instances running in Docker are affected by this.