Login to an add-on domain brings me to the WP login page

Having trouble with the domain mapping plugin. Everything looks good in my Multisite settings but when I try to login to an add-on domain it brings me to the WP login page
I enter credentials and get a warning:
ERROR: Cookies are blocked due to unexpected output. For help, please see this documentation or try the support forums.
I've done everything suggested to allow cookies plus my web host is telling me the plugin is the problem.
Not sure what to try next.