Emma login doesn't work

There's an email marketing software called Emma. I use NextDNS through my router and some of my devices. What I've noticed is that when I try to log in with valid credentials, a POST request is made to https://app.e2ma.net/app2/login/ that always returns 406 when NextDNS is involved. I've confirmed this is a NextDNS issue by connecting through by my smartphone's data as well as through my work MacBook's VPN (which probably resolves DNS some other way).

What could be the issue? I whitelisted e2ma.net already.