User Portal on custom port breaks Clientless VPN

When the User Portal is configured to use a custom port, e.g. 1025, the Clientless VPN tab of the portal is accessible but when clicking on one of the configured connections, the new connection browser window opens and displays the "Connecting" text and then closes itself.

If the User Portal is configured to use the default 443 port, the configured connections work fine.

Thanks,
Adrian