One of the Adium developers reports Microsoft is censoring some messages that are send through Windows Live Messenger (MSN Messenger):
A user on the IRC channel mentioned that he had received that error message. Specifically, he started out asking whether there was a way to “fix sending of URLs through MSN”. I asked for more info, and he revealed that he got the same error message when attempting to send a URL that had a .info domain. I tried a couple such URLs myself, with David's help, and confirmed his report—even http://growl.info/ doesn't work.
The reason MSN gives for this censorship is that it's to protect users from exploits that use certain URLs. The problem with this reason is that any URL could be an exploit URL; filtering by keyword just isn't enough, because the attackers can always invent new filenames. The correct solution is simply to fix the exploits.
There's nothing we can do about this because it's done on the server. This also means that all clients are affected, not just Adium.
We now have a page on our wiki titled MSN Censorship which contains a (probably incomplete) list of strings that are known to get a message blocked. If you include one of these strings in your message, the message will not arrive and you'll get the “connection error occurred” error message in your chat window.
Check out this page to learn more about which words/links Microsoft censors.