You write “Bash seems not to be working for me unfortunatly”. Can you be more specific? OK, the error occurs on Windows, where there is no bash pre-installed. You need to installed it. I recommend cygwin.
Then the cookie-folder of firefox seems to be problematic. Only one false setting of a cookie of some website is enough to let your Duplicati backups fail.
Open your bash-prompt and go into this directory:
cd /cygdrive/c/Users/<your username goes here>/AppData/Roaming/Mozilla/Firefox/Profiles/<your firefox profile goes here>.default-release/storage/default
This command lists all directories in the cookie folder:
ls -d *
Look for similar ones like those:
https+++www.nzz.ch
https+++www.nzz.ch.
Windows Explorer cannot differentiate those two. bash can.
Delete one of them using rm:
rm -r https+++www.nzz.ch
…and your problem is gone.
And as soon as a new version of Duplicati is released, your problem is really gone and you can delete cygwin 