I\'ve installed a RapidSSL certificate, with intermediate certificate, on the site address removed, using DirectAdmin. The RapidSSL installation checker reports a succe
Yes, it's fixable. What you need to do is load the root certificate (that is, the public key of the RapipSSL server--in your case, it's the "GeoTrust Global CA") into the Android device so it knows to trust that certificate authority.
There's a question on the Android StackExchange that suggests multiple ways you can do this.
In that case... unfortunately, the only way to fix it on the server side is to buy an SSL certificate from a provider that is trusted on all the devices you want to use the site. Sadly, this almost always means buying the more expensive SSL certificates from the biggest CAs. (E.g. in my experience, the RapidSSL line was not trusted by handhelds with iOS and WinMobile, but the parent company GeoTrust's base-level certificate was.)
Or... you could provide documentation for your end-users on how to trust the cert on their devices.