ERROR: "No active users found in activity" when workflow fails to assign next activity

Behavior:
Our workflow is unable to assign any user to the next activity. It will display the following exception in the workflow history:

"No active users found in activity with definitionId: ....."

Solution:
Ensure that the user assignment activity has a valid value for this workflow instance. Please refer to the following scenarios on how the user assignments may have been undone;

1. Substitution list
   The user that is assigned over the substitution list is not currently active, or has been deleted.
    
2. User of a group
   The user is either inactive or has been deleted.
    
3. Role:
   The user is either inactive or has been deleted.
    
4. Variable
   The user is either inactive, is misspelled, or the variable is empty.

For assignments over variables, please make sure that the variables have the correct data type.

Note: These are the workflow activities where this error can happen:

• Email Workflow
• General workflow task

KBA is applicable to both Cloud and On-premise Organizations