I've got my Xcode server setup on a mac mini. Downloading the .ipa from https://pandora.local/xcode/ using Itunes works just fine.
I tried installing it directly from the phone from safari. It installs a certificate but when I click install it gives an error, saying that it can't connect to the server.


had the exact same issue and it was a browser caching issue on Safari.
I went into Settings -> Safari -> Advanced -> Website Data, then left swiped on my Xcode server website to reveal the Delete button and clicked on it for just that website.
The next time I browsed to the Xcode server in Safari and attempted to install a build, I received the prompt to install the OTA certificate and when accepted done that I could install applications.