Enhanced transparency, control, and usability in online data management systems are in greater demand due to the challenges users face in tracking who accesses and stores their online personal data. In response, the W3C web standard Solid (Social Linked Data) provides users with data vaults to address these issues. This transformation in how individuals interact with their personal data online through data vaults also presents new challenges in UX design. This paper presents an exploratory usability testing study (n=10), during which participants created and interacted with a data vault within a job application context, aimed at identifying how data vaults impact the user experience. Our study uncovered three key factors introducing UX challenges associated with Solid-enabled data vaults (1) users’ mental models shaping expectations, (2) fears, doubts and the mitigating role of brand trust, and (3) a challenge of explainability rather than usability. These empirically derived factors were analysed through established theoretical frameworks on mental models, technology acceptance, and human-data interaction, enabling the formulation of practical implications and future research topics to address these challenges.