TYPO3 Flow IN-Contraint needs array with Persistence Identifiers as $operand due to an Doctrine bug. You are not able to pass a ArrayCollection or other collection containing objects right now. The following helper would replace an operand containing objects with an array containg the identifiers only!

Example.php

Succeeds:
$portionDemandFrom = $q->matching(
 $q->logicalAnd(
 	$q->lessThanOrEqual('startingFromPersonCount', $this->personCountRange['from']),
 	$q->in('productCategory', $this->generalUtility->prepareOperandForInExpression($this->getProduct()->getCategories()))
 )
)->execute()->getFirst();

Following example would fail:
$portionDemandFrom = $q->matching(
 $q->logicalAnd(
 	$q->lessThanOrEqual('startingFromPersonCount', $this->personCountRange['from']),
 	$q->in('productCategory', $this->getProduct()->getCategories())
 )
)->execute()->getFirst();

Following example would also fail (with different error message:
$portionDemandFrom = $q->matching(
 $q->logicalAnd(
 	$q->lessThanOrEqual('startingFromPersonCount', $this->personCountRange['from']),
 	$q->in('productCategory', $this->getProduct()->getCategories()->toArray())
 )
)->execute()->getFirst();

GeneralUtility.php

<?php
namespace Cf\Shop\Utility;

/*                                                                        *
 * This script belongs to the TYPO3 Flow package "Cf.Shop".               *
 *                                                                        */

use Doctrine\Common\Collections\Collection;
use TYPO3\Flow\Annotations as Flow;

/**
 */
class GeneralUtility {

	/**
	 * @var \TYPO3\Flow\Object\ObjectManagerInterface
	 * @Flow\Inject
	 */
	protected $objectManager;

	/**
	 * @param mixed $operand
	 * @return array
	 */
	public function prepareOperandForInExpression($operand) {
		if ($operand instanceof Collection && 0 < $operand->count()) {
			$collection = $operand;
			$operand = array();
			$persistenceManager = $this->objectManager->get('TYPO3\Flow\Persistence\Generic\PersistenceManager');
			foreach ($collection AS $object) {
				$operand[] = $persistenceManager->getIdentifierByObject($object);
			}

		}
		
		return $operand;
	}
	
}