Hi @Maximilian L ,
From a SharePoint perspective, you can try the following:
1, Check the data types of the fields in the SharePoint list and ensure that they are supported by Power Apps. Reference: https://learn.microsoft.com/en-us/power-apps/maker/canvas-apps/connections/connection-sharepoint-online#lists
2, If the data types are supported, then try refreshing the data source in Power Apps and republishing the form.
3, If the issue persists, try creating a new list and form to see if the problem occurs again. If none of the above works, I suggest you go to the Power Apps community to discuss it and see if the problem occurs in Power Apps.
For your reference: https://learn.microsoft.com/en-us/power-apps/maker/canvas-apps/customize-list-form#customize-a-microsoft-lists-or-sharepoint-form-by-using-power-apps
https://learn.microsoft.com/en-us/power-apps/maker/canvas-apps/sharepoint-form-integration
https://learn.microsoft.com/en-us/power-apps/maker/canvas-apps/sharepoint-scenario-setup
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