Backup hangs in Waiting for Upload to Finish

I have two Ubuntu machines, A and B. Both running the same OS release and the same duplicati release. Both are on the same LAN, talk to the same router which is the gateway, and both use the separate directories on the same backend.

Until very recently backups worth fine on both machines. Machine A still works fine. On machine B I get an infinite hang at either Verifying Backup Data or Waiting for the Upload to finish. B seems to be backing up files, but then hangs on the Waiting for Upload. I have verbose logging on, and at the Waiting for Upload I see a few puts and then nothing.

Any ideas?

Any ideas on how to diagnose

Diagnosis probably depends on the storage type of the destination, and maybe specific details. Any info?