The server is not presenting the complete certificate chain; so if a client does not have the complete certificate chain then the client will result in an error.
Here is an article from Comodo ( a Certificate Authority ) https://support.comodo.com/index.php?/Knowledgebase/Article/View/1019/0/untrusted-certificate-error-on-android
I read that to still be a server side issue. The article starts with "The issue is that the Windows server is not presenting the complete certificate chain . . ." and then proceeds with further background and resolution how-to. Unless I am misunderstanding, this is a server side certificate install issue.