We have two people reporting similar problems but with slightly different error messages.
suggests something pretty low-level (whatever it is). I used to use Internet Explorer to test,
however Microsoft has been killing it off. Can you test with Duplicati CLI tools I mentioned?
An even easier but less effective test is to use Test connection on Destination screen.
That should do a list operation and 1 set (typically 3 files) of get, as end of backup does.
Other harder methods include two traditionally not-Windows programs that Windows ships:
openssl is sometimes good for poking at SSL/TLS issues. curl is aiming a little bit higher.
Grasping at straws, I suppose you could also try allowed-ssl-versions, e.g. hardcode Tls12