I didn’t find any topic here or in GitHub (and I haven’t found in the code), so: is it possible to rename the default Duplicati folder?
I’m asking because the default name (Duplicati backup in Dropbox) has a space, which makes it difficult for multiple operations (outside of Duplicati), generating problems with single quotation marks, double quotation marks, etc.
Instead of Duplicati backup could be Duplicati_backup or even Duplicati only.
You should be able to just manually rename the folder on the destination then update the backup job destination with the new folder name.
I strongly suggest you don’t do that while a backup is in process… 
And where do I set this up?
Because I did not create the “Duplicati backup” folder inside “Applications” folder of Dropbox, I just set it from there:
Ooops - I’m sorry, I misunderstood. I thought you had set “Duplicati backup” as the destination folder.
My GUESS is that Dropbox is creating that folder based on something like the reported application name. If you look at your Dropbox Settings → “Connected apps” you’ll see “Duplicati backup” as the App name.
So it’s probably NOT possible to change that part of the path without changing the name of Duplicati as reported to Dropbox. While I expect this is possible, I believe @kenkendk would be the one to know how hard it would be to do.