Please help us help you and fill where relevant:
LimeSurvey version: [6.15.23]
Own server or LimeSurvey Cloud: own server
Survey theme/template: vanille
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We encountered several issues when installing a question theme through the LimeSurvey interface. The problem was caused by a bug in LimeSurvey: it modified the XML configuration file to match the current version for compatibility purposes. However, this modification made the XML file invalid (duplicate elements, etc.). To fix this, the file must be placed directly on the server.