It was fun Monday, for some crazy reasons AdHoc distribution for iOS devices stopped working with nice little error
Cannot install applications because the certificate for …. is not valid.
Before you go all nuts and start blaming your recent beautiful change you just made, first stop for a second and think if you just improved experience on your phone (nice round red “hung up” button instead of square one I think worth it on its own… along with dozen other changes and fixes) by installing iOS 7.1 update.
If so you found your culprit – in this recent update apps can not longer be downloaded from http:// location. Your download link must use https:// and self-signing certificate would not help you here.
If you have SSL certificate in place, simply adjust your link, if not, then get one and enable https support for your site.