User (one specific) isn't getting notifications

Weird issue with notifications:

  • I have a group with three users
  • group is assigned for receiving notifications
  • in the personal notification settings all topics are activated.

But one user in this group isn’t getting any notification. The other two do.

I can’t see any reason, why this specific user isn’t getting the notifications. Any idea? Some place where I can look for possible errors?

Thanks in advance,