We currently have a User Session type that extends ServiceStack.AuthUserSession. Your recent change to the line:
if (type == null || !type.IsClass || value is string)
have broken the checks on all DateTime types, that actually prevents AuthUserSession to work.
You can verify by using:
ServiceStack.Text.TypeSerializer.HasCircularReferences(new ServiceStack.AuthUserSession())
the function returns False on 5.1.0 and True on 5.1.1 on the DateTime type.