Unable to install app from web on iOS 8
This appears to be a bug with enterprise distribution in iOS 8. If you updated to iOS 8 and had an app with the same bundle identifier installed (i.e. the same app) then you'll be unable to install the app.

A workaround is to change the bundle identifier in the distribution plist. This doesn't require the actual bundle identifier of the app to change, just the distribution plist. Alternatively, you can wait until Apple fixes it.

SE have used the workaround to fix the iOS app beta distribution, as discussed in chat and on the aforementioned meta question.