Symptoms
If a user(USER A) uploads an attachment file, and the user consequently gets disabled or deleted from AAD or AD no other ACTIVE user i.e. USER B, will be able to view or access that attachment file after the fact. They get the error ?User cannot be found?.
Diagnoses
This is a bug in the product.
Resolution
A temporary workaround will be to use the OOTB SharePoint forms in lieu of K2 SmartForms. You can switch this up under Form Setting in the K2 application page.
This will be addressed in Appit 1.5 Update 2 currently scheduled to ship mid-January 2017.