PEBCAK.
In previous attempts, i believe i deleted all advanced options - which include things like the passphrase, database location and whatnot.
This time i ran it keeping all those in place and left the original target URL (i also tried appending the ?username/password which didn’t help - same output):
Target URL: file:///home/kubuntu/Desktop/Backups
Arguments: --version=3
Plus all the pre-existing advanced settings which include the database name etc.
This quickly finished with:
Listing remote folder ...
No remote filesets were deleted
No filesets matched the criteria
Return code: 0
Is it expecting a remote path and is doing something with the URL? For testing convenience, i’m doing this all on a single-machine VM which would not be a typical use-case as obviously we wouldn’t be storing our backups on our desktop 
The log output (set to Live / Information) shows:
Sep 28, 2017 8:25 AM: Email sent successfully using server: smtps://smtp.gmail.com:465
Sep 28, 2017 8:25 AM: No remote filesets were deleted
Sep 28, 2017 8:25 AM: Backend event: List - Completed: (44 bytes)
Sep 28, 2017 8:25 AM: Backend event: List - Started: ()