Syncthing seems to list IPv6 addresses with their scope ID (e.g. %Wi-Fi in the screenshot). These addresses are not supported in popular browsers like Chrome or Firefox. While this doesn't matter much in case of simply listing the address, the problem is real when using the Remote GUI option, as the rendered URLs are broken. In fact, they aren't even treated as proper URLs by the browsers, so there's no copy link option under the right click menu and such.

I think it would be useful to simply filter out scope IDs from the addresses, which should make them work properly in Web browsers.