Icons and domain mapping

we are building the websites of our customers with a multisite wordpress ( bizinthe.cloud)
Our problem: If we are using icons they show up correctly on the subdomain site of the customer (for example https://hansjoergperreten.bizinthe.cloud/) But if we map a domain onto this subdomains the icons are not shown correctly (for example: http://phuket-dreams.com/)
Maybe this is an SSL issue.