Hi @sorrek,
Welcome to the Auth0 Community !
The most probable causes for this issue are that the certificate for your domain has expired or there might be some network-related problems blocking the access to the certificate, as described in this Knowledge Article.
Otherwise, Auth0 only supports Let’s Encrypt or Google Trust for custom domains with managed certificates and an Enterprise subscription is needed in order to manage certificates for custom domains . If the application redirects to an http page instead of https then Auth0 might not be able to find the managed certificate within Let’s Encrypt or Google Trust.
Thanks again for reaching for support and if you have additional questions please do not hesitate to reply to this post.
Best regards,
Remus