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.