I recently had a client where everything was working fine - but they were unable to link to any secure (https) sites when creating entries in Movable Type. Unfortunately, there were no messages to be found, and everything worked just fine - as long as there were no https links in the entry.
To make matters worse, just changing the https link to http made it work just fine - which at first seemed to make no sense whatsoever. It's like that one character put the database over some sort of size limit or something. Alas, even that wasn't it - bunches of (other) characters would work fine, just not those particular characters, making it seem like that site itself was the issue, and that is what eventually led to the answer.