Domain-Mapping plugin causes Mixed Content error during Multi-Domain login

I have an SSL Subdomain Multisite with the Multi-Domains and Domain-Mapping plugins activated.

There are two names in Multi-Domains: domain1 and domain2

After creating sites sub.domain1 and sub.domain2:

The login screen for sub.domain1 (https://sub.domain1/wp-login.php), has normal styled content.

However, the login screen for sub.domain2 (https://sub.domain2/wp-login.php), has unstyled content and 3 console errors:

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Mixed Content: The page at 'https://sub.domain2/wp-login.php?loggedout=true' was loaded over HTTPS,
but requested an insecure stylesheet 'http://sub.domain2/wp-includes/css/buttons.min.css?ver=4.3.1'.

Mixed Content: The page at 'https://sub.domain2/wp-login.php?loggedout=true' was loaded over HTTPS,
but requested an insecure stylesheet 'http://sub.domain2/wp-includes/css/dashicons.min.css?ver=4.3.1'.

Mixed Content: The page at 'https://sub.domain2/wp-login.php?loggedout=true' was loaded over HTTPS,
but requested an insecure stylesheet 'http://sub.domain2/wp-admin/css/login.min.css?ver=4.3.1'.
--

I can fix the errors by deactivating the Domain-Mapping plugin, but I need the Domain-Mapping plugin too.

Re-installing both plugins didn't help.

Any ideas? Access has been granted. Thank you.