Domain mapping SSL SSO insecure endpoint mixed content

Getting a mixed content error on all mapped domain with SSL coming from Domain Mapping SSO endpoint (http://mydomain.net/dm-sso-endpoint/1512169588/?dm_action=domainmap-setup-cdsso).

The main domain and it's original sub-domains are HTTP but can't force https on the whole network because it redirects non-SSL mapped domains to https, which cause an insecure content error.