Sign in hangs when using GNU IceCat

Summary

The sign in doesn't work when using GNU IceCat (while it works in Chromium).

Steps to reproduce

Install GNU IceCat, such as available from GNU Guix (guix install icecat), navigate to gitlab.com and attempt to sign in.

What is the current bug behavior?

The sign in process hangs on the Checking your browser before accessing gitlab.com. dialog. Looking at the Network devtools tab in IceCat, I see that a 503 error is returned for the https://gitlab.com/users/sign_in?redirect_to_referer=yes endpoint.

What is the expected correct behavior?

The sign in should proceed normally.

Output of checks

This bug happens on GitLab.com

Possible fixes

Unknown.