I have 2 Nextcloud 15 servers install on Ubuntu using snaps. One is on a VPS in cloud and uses a Let’s Encrypt certificate that other is on a Proxmox on an internal network and uses a self-signed certificate. Mac and Linux machines with a Safari and Firefox can connect to the web GUI on each with no problem (though they are alerted about the self-signed certificate). Windows machines can connect to the Nextcloud server that uses the Let’s Encrypt certificate but cannot connect to the Nextcloud server that uses the self-signed certificate ( regardless of the browser used – Internet Explorer, Edge, Chrome, Vivaldi, Firefox, Waterfox, Midori). In each case the connection is flagged as insecure and no connection is made – no opportunity to create a certificate exception.