I think I might have had the same problem as you a while ago now that I think of it. It had to do with the permissions on the files it stores your user information in. I forget where they keep it, but it's in the ICQ folder and there should be a folder named the same as your ICQ number.
You can deleted it as all your contacts should be stored on the server. Then just set ICQ up to use your ICQ number again.
That might work for you.