Symptoms

User Task worklist items, when opened by the assigned user, will not display properly and throws the following error:

"List item with id of ___ not found."
 

Diagnoses

When the task is assigned to a user with full control permissions (SharePoint permission level, not within the context of Appit), that user could properly open the worklist item, with the SmartForm displaying correctly. Reassigning, however, to any other user that lacks the "Full Control" permission level, throws the above error when attempting to view the worklist item.

It was discovered that permissions were set at the SharePoint level (specifically, the setting at List(Library) Settings > Advanced Settings > Item Level Permissions: Set to "Read items that were created by the user" and "Create and edit all items") that restricted users from seeing list items they did not create. Because of this setting, any user that is assigned a User Task for the specific list item that does not have read permissions is unable to view the worklist item.
 

Resolution

Changing "Read items that were created by the user" to "Read all items" opens the permissions to allow SmartForms to properly display the list item details.