I experienced this same bug while doing resiliency testing: mid-upload, I restarted the PC. When I logged back in, I was greeted with the error. I refreshed and the error went away!
Then I clicked “Run Now” on the interrupted upload and it finished successfully (+1 on resiliency for Duplicati!). However, this might be related, the progress bar said “0 files” for some reason (one file with two versions):
This is my first go at Duplicati, so that might be normal (i.e., 0 files remaining–uploading last file)?