I'm using the Domain Mapping plugin in version 4 in a WP

I'm using the Domain Mapping plugin in version 4 in a WP 3.6.2 multisite installation (subdomain).

When I login as network admin and try to map a domain to an existing subdomain, I get the error that the domain isn't accessible. The A-record is the same IP for server and domain. It's resolving correctly via ping, so that shouldn't be the issue.

Can you help me with that?

Best regards,
Jan