Message on certificate on Avast web

I have not changed any settings on my computer yet when I log in to the system, I get a message that the certificate has been revoked. This blocks off my internet connection and I cannot access any site. The message reads: Web-shield has been blocked…certificate revoked…idvisitor.slate.com. I understand that this is caused by Avast but I do not understand why. What is a slate problem? Any suggestions on how to remove it since it is very annoying?
