Update2: Irrelevant. Pidgin/libpurple 2.4.3 quickly fixed the issue. Hurray to foss community!

Update: Seems like there'll be a new version soon. From #pidgin at irc.freenode.net:

There are issues with ICQ - ignore the message to upgrade and wait || We know pidgin.im is having difficulties

A few hours ago (01/Jul/2008 ~16:00 UTC) I tried connecting to ICQ using pidgin. Pidgin failed to connect and displayed the following error message:

The client version you are using is too old. Please upgrade at http://pidgin.im/

http://pidgin.im is down at the moment, probably because of the traffic this message caused

So, apparently ICQ have just changed the protocol with no backward compatibility, or at least blocked nonstandard clients. Or maybe it's just a temporary problem?

The only relevant post I've found so far is this Russian post.

I'll update this post when I hear something. Does it happen with other clients as well?