There are several issues to investigate and address:

• Previously when any action execution caused an exception, a message would be sent to all system users. In 5.2.1 this behaviour is changed to only send an error to the user whose action caused the exception. Perhaps depending on the type of action, additional users should be informed such as the superuser, members of the superusers group, or only the administrator of one "tenant" in a multi-tenancy arrangement.
• Perhaps there should be configuration of which users, if any, should recieve messages when exceptions occur for any user.
• Should there be a new method on AppInstanceController to send a message to the administrator of the current user in a multi-tenancy arrangement?
• In response to exceptions - sometimes a message is sent, sometimes only an error notification is displayed. See Andreas' comment below for an assessment from the UX perspective and for how this should work.