We just upgraded our site to version 11 from 10, in local and dev server when we go to / admin it works
Now we deployed same thing in QA environment and when we do /admin it asks to login with popup signin window and we are giving right login and password but the pop up comes again
Try clearing your browser cache. If that doesn't work, resign your macros in System > Macros. Most likely the macro hash changed in your web.config
Looks like this is the same issue as here. Are you providing your AD credentials? Looks like the Windows auth. is configured on this environment and you cannot use the Kentico/forms credentials but your AD credentials.
Please, sign in to be able to submit a new answer.