Gitlab pages with access control redirection is 503 error

Description

We had created an gitlab pages with access control enabled. But the last redirection goes to 503 errors. After refresh the pages, it got 401 error.

What is the http://projects.example.io should point to? If need setup the DNS for projects.example.io, should it be gitlab's server ip or gitlab page's server ip? Could anyone please provide some advice? The gitlab page's setup is simple, but there are many issues caused.

Below are the redirection steps when request the gitlab pages url.

https://group.example.io/project ==> http://projects.example.io/auth?domain=http://group.example.io&state=ZZGBb58zLas_sdzKj88FHA== ==> https://gitlab.example.io/user/sign_in ===> after input the login credentials

https://gitlab.example.io/oauth/authorize?client_id=fcda6fbd0ceb23dcc9546b38db8101426f02d911d985f142193e7939ab790048&redirect_uri=http://projects.example.io/auth&response_type=code&state=ZZGBb58zLas_sdzKj88FHA== ===>

https://group.example.io/auth?code=11d4e52dfbace30d77e50afcca9b8c846487312e364e39231d9b0245afeb2859&state=J8_lKwUoC7sH-RNGiUSxYQ%3D%3D ==>503

Links / references

We had the similar setup refers the below url. https://stackoverflow.com/questions/55108214/gitlab-pages-oauth-redirect-hangs-when-access-control-is-true