This is how it looks from here on preview. Testing what happens when post is made.

Absolute links
http://mudcat.org/threads.cfm Works.
http://www.mudcat.org/threads.cfm Fails. www does not appear to exist on http server.
https://mudcat.org/threads.cfm Works.
https://www.mudcat.org/threads.cfm Works.

Relative links.
mudcat.org/threads.cfm
mudcat.org/threads.cfm
Both fail with part of url duplicated. Probably a bug in text replacement.