ErrorException: Undefined index in array

The following query:
"select c.id
from \\Domain
Cuisine c
left join c.nameTranslation t
left join t.translationValues v
left join v.language l
where v.value = :name
and l.id = :languageId"

Results in a PHP ErrorException in \Doctrine\ORM\Query\SqlWalker line 745:
$assoc = ( ! $relation['isOwningSide']) ? $targetClass->associationMappings[$relation['mappedBy']] : $relation;

When I step through the execution, the array entry: $relation['mappedBy'] gets the following value: "Domain\Translation" in the scope where the error occurs.
$targetClass->associationMappings has the following indices: "translation" and "language" and this leads to a "Undefined index" error and the execution breaks. It means that I cannot execute DQL queries, which is critical for the application to run.

<entity name="Domain\Cuisine" table="Cuisine" repository-class="Infrastructure\Persistence\Doctrine\Repository\CuisineRepository">
<id name="id" type="integer" column="Id">
<generator strategy="AUTO"/>
</id>
<many-to-one target-entity="Domain\Translation" field="nameTranslation">
<join-column name="NameTranslation_Id" referenced-column-name="Id"/>
</many-to-one>
</entity>

<entity name="Domain\Translation" table="Translations">
<id name="id" type="integer" column="Id">
<generator strategy="AUTO" />
</id>
<one-to-many target-entity="Domain\TranslationValue" mapped-by="Domain\Translation" field="translationValues" orphan-removal="true">
<cascade>
<cascade-all/>
</cascade>
</one-to-many>
</entity>

<entity name="Domain\TranslationValue" table="TranslationValues">
<id name="id" type="integer" column="Id">
<generator strategy="AUTO" />
</id>

<field name="value" type="string" column="Value" />

<many-to-one field="translation" target-entity="Domain\Translation">
<join-column name="Translation_Id" nullable="false" referenced-column-name="Id" />
</many-to-one>

<many-to-one field="language" target-entity="Domain\Language">
<join-column name="Language_Id" nullable="false" referenced-column-name="Id" />
</many-to-one>
</entity>

<entity name="Domain\Language" table="Languages" repository-class="\Infrastructure\Persistence\Doctrine\Repository\TranslationRepository">
<id name="id" type="integer" column="Id">
<generator strategy="AUTO" />
</id>
<field name="name" type="string" column="Name" />
<field name="shortIsoCode" type="string" column="ShortIsoCode"/>
<field name="longIsoCode" type="string" column="LongIsoCode"/>
<field name="isDefault" type="boolean" column="IsDefault" />
</entity>