Mixed Content error requested insecure script dm-sso-endpoint

We got the Domain Mapping Tool active on our multisite. But we get the following error message in the console:

“Mixed Content: The page at ‘https://mymappedsite.com/’ was loaded over HTTPS, but requested an insecure script ‘http://mymainsite.com/dm-sso-endpoint/XXXXXX/?dm_action=domainmap-check-login-status&domain=mymappedsite.com’. This request has been blocked; the content must be served over HTTPS.”