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Your LimeSurvey version: LimeSurvey Community Edition Version 6.2.6+230904 
Own server or LimeSurvey hosting: own 
Survey theme/template: Bootstrap Vanilla ( vanilla )
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(Write here your question/remark) I have upgraded from version 5, looks like the template bugs ( I had an Extended template from Vanilla) that was not migrated unfortunately.500 : Erreur interne au serveur Argument 1 passed to Twig\Environment::createTemplate() must be of the type string, null given, called in /var/www/html/limesurvey/application/core/

Did you try to exchange the "config.php" in these themes?
Copy it from one of the core themes and paste it into the "old" theme. (and change the name of the theme there)

 

I think that's config.xml.

Also, maybe related to fhis bug -
bugs.limesurvey.org/view.php?id=19006

Confirming that there is a bug within the survey theme in limesurvey 6.2.6+230904. It throws errors that none of the existing themes are compatible. Creating a new custom theme by extending the provided base theme also has an issue.