I was looking at the screenshots again, and somehow I was thinking this was related to IPv6 support. Could it be that localhost makes it point to the wrong thing?
Could you try --hosturl=http://127.0.0.1:8200 ?
(Also, note that the option is called --hosturl, not --serverurl here)
Is this caused by the update of Duplicati or the re-install of Mint? I can try different flavors of Mint if that makes the difference.