So, sometime in the future, Apple will release iOS 7.1. This new version includes an important change for BetaBuilder users: the Web server that hosts your deployment files now has to be accessible over HTTPS.
Normal HTTP installs will fail. The latest version of BetaBuilder includes a warning to this effect.
Also important – you cannot use a self-signed certificate unless the device trusts the CA it came from. You either need to use a ‘real’ SSL certificate or you need to import and trust the certificate on any devices that will install the app.
So – if you upgrade to 7.1 or are using the beta and cannot install apps with BetaBuilder anymore, this is probably what’s going on.