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:
the function returns False on 5.1.0 and True on 5.1.1 on the DateTime type.
Should now be fixed from this commit, this change is available from v5.1.1 that's now available on MyGet.