For anyone having the same problem, I have another solution:
Do not call the php script in crontab via
/usr/bin/php /home/desyn/public_html/cron/send_scheduled_emails.php
but use a http-request in the crontab file instead:
/usr/bin/curl -s http://example.com/cron/send_scheduled_emails.php > /dev/null
This did the trick for me.