With the launch of iOS 7.1, Apple require all enterprise apps manifest files to be served over HTTPS. Attempts to install through HTTP cause the following message:
The actual page and app IPA don't need to be served on SSL, it's just the manifest file that links to it. Currently you'll have something like this:
But the manifest plist file needs to served over SSL:
You can host the manifest plist file on Dropbox (which will serve it over SSL), but the easiest way is to add an SSL certificate onto your domain. More info over on this stackoverflow question.