[TYPO3-calendar] PHP warning: problem with reminder emails

[tobilot]

Hi List,
Currently, I am testing cal very much to learn all the features and to set up the best calender out there...
At this time I am struggling with all that notification stuff and I have a weared problem with reminder emails:
The emails have a subject, but no content is shown. The emails are sent with the scheduler extension in Typo3 together with a servers cron job -> 
automatically!

I use Typo3 4.7.4 together with cal 1.4.3 (I checked out the latest trunk a few days ago).

When I check the BE log of Typo3, I get following errors for _cli_scheduler (user) at that time the scheduler script should have sent the reminder email:

Core: Error handler (BE): PHP Warning: strtolower() expects parameter 1 to be string, array given in 
.../typo3/typo3_src-4.7.4/typo3/contrib/swiftmailer/classes/Swift/Mime/SimpleHeaderSet.php line 379

Core: Error handler (BE): PHP Warning: strtolower() expects parameter 1 to be string, array given in 
.../typo3/typo3_src-4.7.4/typo3/contrib/swiftmailer/classes/Swift/Mime/SimpleHeaderSet.php line 338

Core: Error handler (BE): PHP Warning: strtolower() expects parameter 1 to be string, array given in 
.../typo3/typo3_src-4.7.4/typo3/contrib/swiftmailer/classes/Swift/Mime/SimpleHeaderSet.php line 334

Core: Error handler (BE): PHP Warning: strtolower() expects parameter 1 to be string, array given in 
.../typo3/typo3_src-4.7.4/typo3/contrib/swiftmailer/classes/Swift/Mime/SimpleHeaderSet.php line 332

some conditions around that:

- I have subscribed one 'unknown' user to one event, he is registered just by the emailadress (for testing purposes).
- No fe user involved for now
- The subscription itself works, emails have content
- The notification emails when the event has been changed are ok as well (have content)
- I have set only the minimum TS settings that are required
- I haven't modified templates at all
- I use the new standard-templates
- The reminder time is set to 30 minutes ahead of the event (default value)
- Database is UTF-8 (as Typo3 4.7 requires that) as shown in phpmyadmin

What could be the reason, that the mail function in Typo3 gives this error?
Is it a bug in cal?

If you need for information please let me know!

THX
Tobias