Can't add https:// domain to domain mapping

Hi there,

I use the Domain Mapping plugin and I can't add the mapped domain with https:// only with http:// . I set the Domain Mapping settings to force HTTPS but this has no effect. If I want to add the domain with https:// I get:

Domain name is unavailable to access. We can't access your new domain. Mapping a new domains can take as lite as .......

The subsite it concerns is https://aprosites.nl/shcn-de/

Thank you for your support,
Tom