On the main admin page, under Top menu navigation, you can an internal link "rockets-forum" and it will add the ".lsp" extension automatically. But if the link is an external URL, like https://reimercast.net, it will still add the ".lsp" to the end which can cause a 404 on the external site.
Should search the link for http:// or https:// and if present, don't add the .lsp. Or maybe just don't add the .lsp at all, since it can cause these problems.
Temporary fix is to add "#" at the end of the URL.