You may try to disable IPS and see if it works.
I can reproduce the problem including the firefox “URL http://detectportal.firefox.com/success.txt issue” when I block all, except selected content
As soon as I change to allow all, except selected content the site is reachable again.
If you like to allow specific sites you may add a custom category. Then you may use block all and just add sites you like to be allowed to the custom category.