Hi Luis,
In Manual Task, "Assign to" field should be always assigned to one user while assigning it manually. Alternately, you could assign to a role(with multiple users) as explained in
here.
The error gets reflected only in LIVE, because in TEST mode, the task is assigned only to process initiator.
If you want to notify multiple users for the task, you can include their mail-ids in E-mail step configurations.
Hope this answers query.
Regards,
Murali
Fujitsu RunMyProcess Support