"Class does not exist. Reflection failed." with lazy loading and object storages

I'm upgrading a website from TYPO3 8 to TYPO3 9

All the code was working well with TYPO3 8.
Now I have to adapt a lot as it came from even earlier TYPO3 (TCA, doctrine, ...) and throws some errors.

At the moment I have the problem for some pages I only get this error (in slight modifications):

(1/2) #1278450972 TYPO3\CMS\Extbase\Reflection\Exception\UnknownClassException

Class VendorName\VendorExtensionName\Domain\Model\TYPO3\CMS\Extbase\Persistence\ObjectStorage does not exist. Reflection failed.

I assume it is triggered by this code:

<?php
namespace Citkomm\CitkoEgovserviceLight\Controller;

class AnsprechpartnerController extends \TYPO3\CMS\Extbase\Mvc\Controller\ActionController {

    /**
     * ansprechpartnerRepository
     *
     * @inject
     * @var \VendorName\VendorExtensionName\Domain\Repository\AnsprechpartnerRepository
     */
    protected $ansprechpartnerRepository;

    [...]

    /**
     * action showDetail
     *
     * @return void
     */
    public function showDetailAction() {
        $pids = $this->settings['pids'];
        $this->settings['ansprechpartner'] = explode(',', $this->settings['ansprechpartner']);
        foreach ($this->settings['ansprechpartner'] as $uid) {
            $person = $this->ansprechpartnerRepository->findByUid($uid);  

    [...]

as this last line is in the debug stack.

The extension has 8 kinds of records which relate to each other. I assume because of this the relations are defined lazy and object storages are used.

<?php
namespace VendorName\VendorExtensionName\Domain\Model;

/**
 *
 * @package vendor_extension_name
 *
 */
class Ansprechpartner extends \TYPO3\CMS\Extbase\DomainObject\AbstractEntity {

[...]


/**
 * Organisationseinheit
 *
 * @var \TYPO3\CMS\Extbase\Persistence\ObjectStorage<\VendorName\VendorExtensionName\Domain\Model\Organisation>
 * @lazy
 */
protected $organisationseinheit;

/**
 * Dienstleistungen
 *
 * @var \TYPO3\CMS\Extbase\Persistence\ObjectStorage<\VendorName\VendorExtensionName\Domain\Model\AnsprechpartnerDienstleistung>
 * @lazy
 */
protected $dienstleistungen = NULL;

[...]

But here the order of the mixed up classes is reversed.
Nonetheless this might be the reason for the mixing/concatenating of the existing namespaces VendorName\VendorExtensionName\Domain\Model[\Ansprechpartner] and
\TYPO3\CMS\Extbase\Persistence\ObjectStorage to the strange class name
VendorName\VendorExtensionName\Domain\Model\TYPO3\CMS\Extbase\Persistence\ObjectStorage, which of course does not exist.