Go to Configuration>Auth>L3 profile>Captive portal profile (choose the one u are working with) - and be sure the choose the right user role (not a user role with captive) - for example : guest.
if u choose the inital role that the captive giving to your users as the same role that showing them the captive = LOOP.
here is an exmaple.
here is a user role with captive configured in it: (example)
and here is the captive profile - with the right role to give to users after they entering good user&password: (l3 profile>captive example)