This happened to me recently and was driving me nuts!
It also cost me several new user sign-ups … 
Spot where I fixed it! 
Use the rails console to unpause sidekiq. (This is only a temporary fix though)
Check your logs, it’s likely that you have an error. I was seeing Access Denied like messages. Check your S3 backup process if you have one. For some reason there must have been an update to it at some point that required additional permissions on Amazon S3. A broader policy on AWS S3 fixed it for me.
YMMV