Symptoms

After adding the app to the new app store the product fails at both the root level (was working before the app add) and the second web app. The error is as follows:

Clicking on K2 for SharePoint errors 'This page can't be displayed' with 3 bulleted items:
•Make sure the web address http://k2dev.denallix.com is correct
•Look for the page with your search engine.
•Refresh the page in a few minutes.
 

Diagnoses

WIt was determined that there was an issue pinging the app domain. 
 

Resolution

For example, ping Apps-12345678ABCDEF.contosoapps.com
As per: https://technet.microsoft.com/en-us/library/FP161236.aspx
After working with the Active Directory admin, we resolved the app domain issue an was able to add the app successfully.

Please also use a non-System account when adding/registering the app as per:
http://blogs.msdn.com/b/how24/archive/2013/06/14/prepare-your-sharepoint-2013-farm-for-app-development-and-debugging.aspx

It recommends:
"2. Don’t use Farm admin (System Account) to deploy and test apps
SharePoint will not allow you to install nor uninstall apps using the farm account (System Account)."

http://help.k2.com/onlinehelp/K2ForSharePoint/ICG/4.6.9/default.htm_Install_Topics/Maintenance/Troubleshooting.htm
Section: "K2 App Install with the System Account"