Getting “Cannot install applications because the certificate is not valid” error on iOS?
With the launch of iOS 7.1, Apple requires the enterprise apps manifest file (.plist) to be served over HTTPS. Attempts to install through HTTP cause the above message.
A change was introduced to iOS as of iOS 7.1 that requires OTA application installation to take place over HTTPS (rather than HTTP), and no provision exists to allow the user to continue to accept the old behavior (i.e., to permit application installation over HTTP). It is my understanding that this was done as a security enhancement.
In order to do application installations on iOS 7.1 from Application Center, you would need to configure your Application Center to use HTTPS rather than HTTP, using a certificate that will be accepted as valid by your iOS device – that is, either issued by a recognized public CA, or issued by a private CA that you have set up your mobile devices to accept.