[TYPO3-english] cronjob: Unable to determine path to entry script

[Sascha Orf]

Hello,

hopefully someone can help me. I'm trying to run a cronjob but this ends up with:

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Status: 500 Internal Server Error
X-Powered-By: PHP/7.1.10
Content-Type: text/plain

Unable to determine path to entry script.
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This exception is thrown in SystemEnvironmentBuilder.php. The function checkMainPathsExist() gets the path to the 'typo3' directory, where index.php is. 
What could be wrong here, which path would be correct or which one is the entry script?

Thank you so much for every hint!