SSL invalid certificate issue

I have encountered an SSL issue in my mobile Android app at https://median.co/share/pwnkow. Whenever a page loads, I receive a popup saying "Invalid SSL certificate." I have verified that the SSL certificate is valid for the specified time period. I have also fixed the URL from http:// to https:// for the homepage, but I am still getting the "Invalid SSL certificate" error.

Please help me to how to resolve the SSL issue on my android app.