Customer is not sure when this started (no upgrades, or App ID Password changes, etc.), but they went to review their inventory policies and they all bring up this error page.
Permissions issue on either Web.Config or the Web folder
For this instance, the Inventory\Web folder was missing User permissions. By default, the Altiris folder has Authenticated User permissions, and this was missing from all of the Altiris folders and most were replaced with the normal Users folder. The Web folder did not have Users or Authenticated Users. Adding permissions there resolved this issue as well. We decided that we would fix this issue by replicating permissions from the Altiris folder, Down which also removed Users' permissions which were not needed. From the Altiris Folder > Permissions, click the Security Tab > Advanced > Change Permissions > Replace all child object permission entries. Last, it is necessary to restart IIS after making these changes.