Setting up destination:
Storage type: WebDAV
Use SSL: (unticked)
Server and port: https://webdav.icedrive.io (port is unspecified)
Path on server: /Duplicati
Username: danvondra@protonmail.com
Password: (value obtained from icedrive access settings → WebDAV)
Edit: No firewall running. Pinging webdav.icedrive.io works fine.
Running on Linux Mint 20.3.
Edit 2: Also turned off VPN. Still fails.