TYPO3 Core

While creating a Reference of a PDF i get following ERROR and the Info appears ERROR 500:

Core: Exception handler (WEB): Uncaught TYPO3 Exception: #1476107295: PHP Catchable Fatal Error: Method TYPO3\CMS\Core\Imaging\ImageManipulation\CropVariantCollection::__toString() must return a string value in /html/typo3/typo3_src-8.6.1/typo3/sysext/backend/Classes/Form/Element/ImageManipulationElement.php line 289 | TYPO3\CMS\Core\Error\Exception thrown in file /html/typo3/typo3_src-8.6.1/typo3/sysext/core/Classes/Error/ErrorHandler.php in line 113. Requested URL: http://www.myprivatedomain.com/typo3/index.php?ajaxID=%2Fajax%2Frecord%2Finline%2Fcreate&ajaxToken=16e4515f596df5de16be5b1cf2e68627da325634

Also in install-Tool, there is following Error at Image Testing ( Read PDF / Read AI ):
Image generation failed
ImageMagick / GraphicsMagick handling is enabled, but the execute command returned an error. Please check your settings, especially ['GFX']['processor_path'] and ['GFX']['processor_path_lzw'] and ensure Ghostscript is installed on your server.

the path-variables are correct and ghostsctipt is installed

TYPO3 Version: 8.6.1