Nextcloud 11: no mails are sent even though it is configured

[BenS89]

Steps to reproduce:

• Use activity app 2.4.1 with Nextcloud 11.0.1
• Activate all notification events (mail and stream) in activity personal settings panel
• Check "send out mails hourly"
• Check "notify about own activities via mail"
• Configure and test mail server in php send mode within Nextcloud admin panel
• Configure your system cron service to run hourly
• Do some things --> Some CRUD operations on your files

Expected behavior:
After one hour, a mail is sent out with all personal changes and with all new shares by the activity app.

Actual behaviour:

• Simply nothing happens.
• Logs are empty
• Sending a test mail from the admin panel is successful.

Enabled apps:
Calendar 1.5.0
Contacts 1.5.3

PHP-Version: 7.1
OS: Embedded QNAP-Linux