[TYPO3-project-seminars] Error after installation cal_ts_service, oelib & seminars

[Robin Clemens]

Hey there,
i've installed on Typo3 4.1.5 the actual versions of the extensions  
cal_ts_service, oelib and seminars. Then i have added the include  
static cal_ts_service. After this i've become this error message
Warning: require_once(PATH_tslibclass.tslib_fe.php) [function.require- 
once]: failed to open stream: No such file or directory in/kunden/ 
213437_42549/cms/typo3/typo3/typo3conf/ext/oelib/ 
class.tx_oelib_templatehelper.php on line 25

Fatal error: require_once() [function.require]: Failed opening  
required 'PATH_tslibclass.tslib_fe.php' (include_path='.:/usr/local/ 
lib/php:/kunden/213437_42549/cms/typo3/typo3/typo3conf/ext/cal/res/ 
PEAR/') in/kunden/213437_42549/cms/typo3/typo3/typo3conf/ext/oelib/ 
class.tx_oelib_templatehelper.php on line 25

This is not only displayed after adding the include static, after  
saving some content too.

does anyone knows something about this?

Thanks!
robin