Could you double-check if you've followed all the steps here, especially with regards to the supported format?
Certificates and authorities can be troublesome – sometimes locally everything works, and while deploying you get these issues. I'd second Eline's advice, and alternatively do a check on the certificate itself with an external tool, such as https://www.digicert.com/help/. If all is in order, I'd contact Mendix support – they are the only ones who can debug the environment at the necessary level (Java).