What does this warning signify?

Warnings: [
Expected there to be a temporary fileset for synthetic filelist (9, duplicati-i9b4ba4f6462b46b6a24f5703aac2f6e4.dindex.zip.aes), but none was found?

Usually it signifies Duplicati got interrupted in some way during a previous backup but isn’t usually anything to be concerned about.

Here are two posts that explain it a little more: