ISSUE SUMMARY:
If the administrator of an event sends an email to all attendees, the subject of the email is missing although I entered one in the respective field.
There is a notification in the front-end when you click on this earth icon and this notification is completely empty.

Please log on to our dev site. I already added your user to an event and send a email to all attendees. You can see that the notification is blank.
If you logon to the back-end, you can check the mail queue and see that the subject in the emails is missing.

Thanks

Hi,

please check it, at your language file the text not valid for event subject. to tackle this issue I put joomla language overrides for
COM_COMMUNITY_EVENT_SENDMAIL_SUBJECT
I changing the text from

Neue Aktualisierungen für %1$s

Das Event {event} hat Dir eine E-Mail gesendet: {title}

The {event} event sent you an email: {title}

Thanks Albertus, problem solved.