Seems like people have had this problem in the past, and now it's back at least for me in version 1.2.1. I input my ICQ number as screen name, and as I try to connect, I get this error message (I'm not using the English version so this is my own translation):
"Unable to login: Unable to login as user ###-###-### (my ICQ number) because screen name is erroneous. Screen names should either start with a letter and only contain letters, numbers and spaces or only contain numbers."
Got no problem logging into ICQ on WinGaim 1.2.1, altho 1.2.0 was a problem for me.