I fixed it. Here’s what I had to do:
cd /var/discourse
./launcher ssh app
rails c
Sidekiq.redis { |r| puts r.flushall }
Then I had to exit the container and ./launcher restart app
That cleared the Sidekiq Redis queue and everything started back up.
If I had to guess, I upgraded Discourse when there was a backup job in the queue in Redis and somehow, upon restarting the app, the name of a class or something associated with Redis, Sidekiq, or backing up had changed, so the item remaining in the Redis queue was invalid and triggering an error in the logs. That’s total speculation.