Hello @Maximilian Goch,
Thank you for reaching Microsoft!
Prior to modifying the content of an event designated as an online meeting, it is crucial to retrieve the body property, make the necessary adjustments to the content, and ensure the online meeting blob is retained. Accidental deletion of the meeting blob from the body can result in the disable of online meeting. Please refer the below screenshots and document.
Documentation reference: https://learn.microsoft.com/en-us/graph/api/event-update?view=graph-rest-1.0&tabs=http#notes-for-updating-specific-properties
Hope this helps.
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