A lot of times as an Enterprise Portal developer, i have faced this problem of URL property error. This is a problem that persisted in Microsoft Dynamics Ax 2009 and in Ax2012.
When we hit the ellipses button while setting the URL for a URL Menu Item, the Ax application fails to open the Enterprise Portal website folders through which we add a URL to the Enterprise Portal .aspx pages. The error is as showcased in the screenshot below.
1. To resolve the above mentioned issue, download the hotfix through the link mentioned below.
2. Install the package and try to navigate to the web folders. It will show the browse web folders window. On hitting the ellipses button, one should get the Browse for Files or Folders window to navigate to the .aspx pages that represent the Enterprise Portal web pages.