Do you know if it was ever large? Depending on retention settings, you can look at old backup logs.
Key numbers are SizeOfExaminedFiles (was it about 400MB?) from source and BackendStatistics KnownFileSize.which is the oddly low size on the home screen. It’s typically closer to source size…
Incomplete source folder tree #3063 was the only report I could readily find, and the guess was some access problem. I can get a similar icon if my user doesn’t have access to the folder. Duplicati defaults on Windows to running as you not SYSTEM. Can you get into those folders in File Explorer?
Verifying backend files says what Verify does. It verifies sample remote files match database records, which does mean they’re available to restore, but more importantly that the contents are as expected. Simple sanity-check of existence (and maybe size) of expected backup files is done on every backup.