workflow designer task "could not find any user for assignment"

I am getting the follow error and I am unsure how to resolve it. See below:
 

Dear Controller, 
an error occurred in workflow [Workflow Name]. The activity [Activity Name] could not find any user for assignment. The next task was assigned to controllers. 
In order to get more information on the error message and resolve the issue, please follow the link: 

[Link] 

When I review the WF history, I see the user assignments. All three users are active and have Named licenses. How do I fix this?

Thanks,

Hi Waylon, i think the problem is not that the task was not assigned to the users, but the task ran into a timeout and this timeout was not yet assigned to a user to check and therefore the error email was generated.

Please check what happens if the users do not take care of the task. 

Best regards
Alex