So, on a forum, when you restrict access to a specific group and later on the group is deleted, firstly, it generates an error when you try to access the subscription panel, and more problematic, it generates an error, and it stops the mod_forum/cron/task blocking the notification distribution. 

Step to reproduce:

• In a new course, enroll some students and make a group with them
• create a new forum and restrict the access to the related group
• force subscription to all
• add a discussion and see that all notifications are sent successfully
• remove the group 
• see that trying to access the subscription panel generate an error
• add a discussion and see that the task mod_forum/cron/task is returning an error too