Just tried switching to Gitlab Next and ran into a bad SSL certificate error message. The cert being served seems to be only valid for *.gitlab.io and gitlab.io
Here is the full error message from Firefox -
Websites prove their identity via certificates. Firefox does not trust this site because it uses a certificate that is not valid for next.gitlab.com. The certificate is only valid for the following names: *.gitlab.io, gitlab.io
Error code: SSL_ERROR_BAD_CERT_DOMAIN