This error is specific to older Android versions (including 4.4.4 and older). The root certificate on the device will need to be updated. If the device does not have a password or pin to lock the device please set a security code on the device prior to performing these steps.  
  1. Access https://www.digicert.com/CACerts/DigiCertGlobalRootG2.crt on the device.
  2. Click "Download" when prompted to download a file.
  3. When prompted to name the certificate, keep the default values and select "OK."

The trusted certificate should now be installed. To confirm, go to Settings > Security > Trusted Credentials (User tab).  

If the error "Certificate could not be installed because it cannot be read" is received, follow the steps below:
  1. Go to Settings > Security > Install from Storage
  2. Locate certificate file
  3. Click Install