Debugging errors like "An error occurred while trying to call Vendor\ExtKey\Controller\ModelController->showAction() " in extbase extensions

ModelController.php

	/**
	 * A special action which is called if the originally intended action could
	 * not be called, for example if the arguments were not valid.
	 *
	 * The default implementation sets a flash message, request errors and forwards back
	 * to the originating action. This is suitable for most actions dealing with form input.
	 *
	 * We clear the page cache by default on an error as well, as we need to make sure the
	 * data is re-evaluated when the user changes something.
	 *
	 * @return string
	 */
	protected function errorAction() {
		echo 'ErrorAction';
		$this->clearCacheOnError();
		if ($this->configurationManager->isFeatureEnabled('rewrittenPropertyMapper')) {
			echo 'Rewritten Property Mapper';

			echo '<pre>';
			foreach ($this->arguments->getValidationResults()->getFlattenedErrors() as $propertyPath => $errors) {
				foreach ($errors as $error) {
					$message .= 'Error for ' . $propertyPath . ': ' . $error->render() .
						PHP_EOL;
				}
				echo 'Error: ' . $message;
	
			}
			echo '</pre>';

			$errorFlashMessage = $this->getErrorFlashMessage();
			if ($errorFlashMessage !== FALSE) {
				$errorFlashMessageObject = new \TYPO3\CMS\Core\Messaging\FlashMessage(
					$errorFlashMessage,
					'',
					\TYPO3\CMS\Core\Messaging\FlashMessage::ERROR
				);
				$this->controllerContext->getFlashMessageQueue()->enqueue($errorFlashMessageObject);
			}
			$referringRequest = $this->request->getReferringRequest();
			if ($referringRequest !== NULL) {
				$originalRequest = clone $this->request;
				$this->request->setOriginalRequest($originalRequest);
				$this->request->setOriginalRequestMappingResults($this->arguments->getValidationResults());
				$this->forward($referringRequest->getControllerActionName(), $referringRequest->getControllerName(), $referringRequest->getControllerExtensionName(), $referringRequest->getArguments());
			}


			$message = 'An error occurred while trying to call ' . get_class($this) . '->' . $this->actionMethodName . '().' . PHP_EOL;
			return $message;
		} else {
			// @deprecated since Extbase 1.4.0, will be removed two versions after Extbase 6.1
			$this->request->setErrors($this->argumentsMappingResults->getErrors());
			$errorFlashMessage = $this->getErrorFlashMessage();
			if ($errorFlashMessage !== FALSE) {
				$errorFlashMessageObject = new \TYPO3\CMS\Core\Messaging\FlashMessage(
					$errorFlashMessage,
					'',
					\TYPO3\CMS\Core\Messaging\FlashMessage::ERROR
				);
				$this->controllerContext->getFlashMessageQueue()->enqueue($errorFlashMessageObject);
			}
			$referrer = $this->request->getInternalArgument('__referrer');
			if ($referrer !== NULL) {
				$this->forward($referrer['actionName'], $referrer['controllerName'], $referrer['extensionName'], $this->request->getArguments());
			}
			$message = 'An error occurred while trying to call ' . get_class($this) . '->' . $this->actionMethodName . '().' . PHP_EOL;
			return $message;
		}
	}

Got the error now... BUT! If I clean my cache from Backend (the 3) then everything works as expected... how can this be a validation problem? ANy Idea?

Thank you so much! Saved my day!

Very helpful. Thanks.