This is because Yahoo! will no longer be providing chat to European users anymore. They have agreed with Lycos to point all European users to Lycos chat instead.
Lycos chat is nowhere near as good as Yahoo! despite the fact they say its the "most advanced chat community in europe". It's web based only, so no pretty interfaces, slow loading graphics (each room change needs new graphics). The adverts are in-vue (they appear over the chat room and obstruct your view) and there are also adverts that appear as normal chat comments.
When I say Europe, I mean: UK & Ireland, France, Germany, Italy and Spain
This change is currently being "beta" tested, you can go here to find out more:
http://uk.docs.yahoo.com/chat/chatbylycos.html#1
The preview ends after 3 weeks, its been about 1 week already, when the preview ends, the changeover is permanent, no more Yahoo! UK & Ireland provided Chat.
To read my full comments on what I experienced with Lycos, read here:
www.helpbytes.co.uk/lycos.php