Are your SSL certificates SHA-256 and what is their compatibility?
Certificates issued with the SHA-256 hashing algorithm have support on most modern operating systems. Some older systems only support hashing algorithm and as such, SSL certificates will show as invalid for these users.
As a general rule, SHA-256 is supported on OS X 10.5+ and Windows XP SP3+. For a detailed guide, please see below.
Operating System Support
|Apple OS X||10.5+|