Hi CPichler, here you have the link that has helped me to resolve the same issue than yours: https://help.nextcloud.com/t/enable-notification-emails-activity-app-not-work/124699/2
Brief, it seems that checkbox only works to change from “yes” to “no”. If you want to “enable” it, you have to issue a SQL statement.
I have to say that this issue is not resolved in NC22. I’ve it updated from 20 to 21 and then to 22 and nothing changes…
Regards!