Invalid Certificate Received From Server

WebAPI (built with .Net) is hosted on a server and called from iOS and Android apps using HTTPS. No changes has been made in certificates or otherwise. iOS app works fine, web app

Solution 1:

Solved and the app on those older Android versions works now. I had to disable COMODO root certificate COMODO RSA Certification Authority with SHA1 key afe5d244a8d1194230ff479fe2f897bbcd7a8cb4 in certificate manager everywhere it was. Apparently windows update added this and it was somehow conflicting and making an intermediate certificate not being sent from the server (in the second certificate chain, the first chain was ok). Normally if an intermediate certificate is not sent, clients download it from the issuer but Android mono framework apparently doesn't do that.

Thank you Adm Selec https://community.qualys.com/thread/15295 for the solution.