[Typo3] Cannot use string offset as array.... class.tslib_fe.php on line 1328

[Markus Stauffiger]

Hi Typo3-Gurus

On one page I have the following error appearing now and then, I've seen 
some postings on this subject, but I couldn't find any hint...

Fatal error: Cannot use string offset as an array in 
/home/etzel-immo/public_html/typo3_src/typo3/sysext/cms/tslib/class.tslib_fe.php 
on line 1328

Do you have any idea what's causing this error? And why only sometimes?

Any hint will be appreciated!

Thanks
Markus