TYPO3 Core

But I can't that name in lowercase anywhere in the source code.. Could you post a stack trace, please?

Thanks for your quick answer. Here is the stack.

#0 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(171): TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\CMS\Form\...')
#1 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(151): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->createElement('CHECKBOXGROUP', Array)
#2 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(114): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->addElement(Object(TYPO3\CMS\Form\Domain\Model\Element\FieldsetElement), 'CHECKBOXGROUP', Array)
#3 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(94): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->setElementType(Object(TYPO3\CMS\Form\Domain\Model\Element\FieldsetElement), 'CHECKBOXGROUP', Array)
#4 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(204): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->getChildElementsByIntegerKey(Object(TYPO3\CMS\Form\Domain\Model\Element\FieldsetElement), Array)
#5 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(178): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->reconstituteElement(Object(TYPO3\CMS\Form\Domain\Model\Element\FieldsetElement), Array)
#6 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(151): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->createElement('FIELDSET', Array)
#7 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(114): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->addElement(Object(TYPO3\CMS\Form\Domain\Model\Form), 'FIELDSET', Array)
#8 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(94): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->setElementType(Object(TYPO3\CMS\Form\Domain\Model\Form), 'FIELDSET', Array)
#9 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(204): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->getChildElementsByIntegerKey(Object(TYPO3\CMS\Form\Domain\Model\Form), Array)
#10 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(178): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->reconstituteElement(Object(TYPO3\CMS\Form\Domain\Model\Form), Array)
#11 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Domain/Factory/TypoScriptFactory.php(60): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->createElement('form', Array)
#12 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Controller/FormController.php(166): TYPO3\CMS\Form\Domain\Factory\TypoScriptFactory->buildModelFromTyposcript(Array)
#13 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Controller/FormController.php(132): TYPO3\CMS\Form\Controller\FormController->renderForm()
#14 /var/www/virtual/host/htdocs/Core/typo3/sysext/form/Classes/Controller/FormController.php(119): TYPO3\CMS\Form\Controller\FormController->execute()
#15 /var/www/virtual/host/htdocs/Core/typo3/sysext/frontend/Classes/ContentObject/ContentObjectRenderer.php(727): TYPO3\CMS\Form\Controller\FormController->cObjGetSingleExt('FORM_INT', Array, '10', Object(TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer))
#16 /var/www/virtual/host/htdocs/Core/typo3/sysext/frontend/Classes/ContentObject/ContentObjectRenderer.php(682): TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer->cObjGetSingle('FORM_INT', Array, '10')
#17 /var/www/virtual/host/htdocs/Core/typo3/sysext/frontend/Classes/ContentObject/ContentObjectArrayContentObject.php(49): TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer->cObjGet(Array)
#18 /var/www/virtual/host/htdocs/Core/typo3/sysext/frontend/Classes/ContentObject/ContentObjectRenderer.php(880): TYPO3\CMS\Frontend\ContentObject\ContentObjectArrayContentObject->render(Array)
#19 /var/www/virtual/host/htdocs/Core/typo3/sysext/frontend/Classes/Controller/TypoScriptFrontendController.php(3461): TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer->COBJ_ARRAY(Array)
#20 /var/www/virtual/host/htdocs/Core/typo3/sysext/frontend/Classes/Controller/TypoScriptFrontendController.php(3404): TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController->INTincScript_process(Array)
#21 /var/www/virtual/host/htdocs/Core/typo3/sysext/cms/tslib/index_ts.php(221): TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController->INTincScript()
#22 /var/www/virtual/host/htdocs/Core/index.php(42): require('/var/www/virtua...')
#23 {main}
Fatal error: Class '\TYPO3\CMS\Form\Domain\Model\Element\CheckboxgroupElement' not found in /var/www/virtual/host/htdocs/Core/typo3/sysext/core/Classes/Utility/GeneralUtility.php

And here the relevant part from the form setup:

60 = FIELDSET
60 {
    legend {
        value = Anfahrt
    }
    10 = CHECKBOXGROUP
    10 {
        class = fieldset-subgroup
        legend {
            value = Anfahrt
        }
        name = anfahrt
        10 = CHECKBOX
        10 {
            label {
                value = Zug
            }
        }
        20 = CHECKBOX
        20 {
            label {
                value = Bahn
            }
        }
        30 = CHECKBOX
        30 {
            label {
                value = Flugzeug
            }
        }
    }
}

Let me know if you need something more.

Fixed with #40850