"err_certificate_invalid" for only one host, on only one device, but in all browsers
Your certificate is issued by StartCom. The company has recently endured a number of scandals where they broke the rules governing certificates.

This means that for example new versions of Firefox does not trust StartCom certificates issued after the 21st of October 2016. Your certificate is from 2017.

Your problems are probably caused by this. The iPad likely has newer software/certificate updates than your Windows or Android device.

In time expect that no one will accept your StartCom certificate.

I suggest replacing the certificate with a new certificate!