Hi Kristof, have you upgraded recently from Mx8 to Mx9? And did you already follow the instructions to right click and update?
It looks like the properties configured in your widgets don’t match the settings.json in your /theme folder. This is usually caused by an incorrect Atlas UI version. Mx9 is not compatible with Atlas 2, so make sure you update to Atlas 3. Use the doc’s for this one: https://docs.mendix.com/refguide/moving-from-atlas-2-to-3.
Mendix 9 works just fine with Atlas 2, as long as you don't update the theme. As soon as you switch to Atlas 3, you need a fresh theme folder. I usually have an empty project of the same Mendix release at hand to copy the theme folder from. The migration instructions also provide a downloadable zip you may use. Before you copy, rename the existing folder so you can rescue your current theme. Note you cannot just copy an Atlas 2 theme into an Atlas 3 theme! Follow the instructions to migrate your app properly.