When do you get that error message? Is it just when you enable the SCLogin module and navigate to any page, or when you try to login?
If it's when you login, then that's likely a Joomla issue and not something specific to the SCLogin module. Our SCLogin module doesn't actually perform the login, we simply hand off the user credentials to Joomla itself. If you're having that issue during login, I'd recommend trying the standard Joomla login (mod_login) module and see if the issue occurs there as well.
As for the Authentication - Cookie plugin, I really don't know what that is. It's not a standard Joomla plugin, as far as I know. You'd need to contact their developers if it is indeed giving you issues.
Thanks,
Alex