In version 10 of Forms you did not need to add each user that would participate in a Forms process to the Access Rights section of the Forms process. Instead, you could just assign them to the User Task and they would be given the necessary rights to complete their tasks.
In Forms 11.0 if a user that is assigned a user task is not assigned access rights to the Forms process, and a form that was created with the Modern Forms designer is assigned to the user task, they will be able to see the task in their Inbox, but after clicking on it, it will show empty where the form will usually be displayed. If you check F12 Dev Tools-> Console and expand the messages it says: