A user was unable to connect to a Zoom call. It would spin trying to connect for quite a while, then finally display a certificate error:
Some googling finally led me to check for a proxy. In Internet Explorer 11 Options, click the Connections tab, then LAN settings. Sure enough, a proxy was configured, if I recall correctly to the local machine (127.0.0.1). This is possibly a remnant of her old Juno email client, which was notorious for forcing a proxy:
Once I unchecked “Use a proxy server”, Zoom was immediately able to connect.