The tx_oelib_Model_FrontEndUser with the UID 49 either has been deleted (or has never existed), but still is accessed.

#0 /var/www/oeh/htdocs/nachhilfe/typo3conf/ext/oelib/class.tx_oelib_Object.php(78): tx_oelib_Model->get('name')
#1 /var/www/oeh/htdocs/nachhilfe/typo3conf/ext/oelib/class.tx_oelib_Object.php(90): tx_oelib_Object->getAsString('name')
#2 /var/www/oeh/htdocs/nachhilfe/typo3conf/ext/oelib/Model/class.tx_oelib_Model_FrontEndUser.php(72): tx_oelib_Object->hasString('name')
#3 /var/www/oeh/htdocs/nachhilfe/typo3conf/ext/seminars/pi1/class.tx_seminars_pi1.php(1367): tx_oelib_Model_FrontEndUser->getName()
#4 /var/www/oeh/htdocs/nachhilfe/typo3conf/ext/seminars/pi1/class.tx_seminars_pi1.php(782): tx_seminars_pi1->setOwnerDataMarker()
#5 /var/www/oeh/htdocs/nachhilfe/typo3conf/ext/seminars/pi1/class.tx_seminars_pi1.php(353): tx_seminars_pi1->createSingleView()
#6 [internal function]: tx_seminars_pi1->main('', Array)
#7 /var/www/oeh/htdocs/nachhilfe/typo3/sysext/cms/tslib/class.tslib_content.php(6353): call_user_func_array(Array, Array)
#8 /var/www/oeh/htdocs/nachhilfe/typo3/sysext/cms/tslib/content/class.tslib_content_user.php(56): tslib_cObj->callUserFunction('tx_seminars_pi1...', Array, '')
#9 /var/www/oeh/htdocs/nachhilfe/typo3/sysext/cms/tslib/class.tslib_content.php(927): tslib_content_User->render(Array)
#10 /var/www/oeh/htdocs/nachhilfe/typo3/sysext/cms/tslib/class.tslib_fe.php(3323): tslib_cObj->USER(Array)
#11 /var/www/oeh/htdocs/nachhilfe/typo3/sysext/cms/tslib/class.tslib_fe.php(3261): tslib_fe->INTincScript_process(Array)
#12 /var/www/oeh/htdocs/nachhilfe/typo3/sysext/cms/tslib/index_ts.php(465): tslib_fe->INTincScript()
#13 /var/www/oeh/htdocs/nachhilfe/index.php(83): require('/var/www/oeh/ht...')
#14 {main}