Hello Patrick Schärer,
Here is what you can try:
1. Remove all references to Telerik controls.
2. Create a new folder in your project, e.g. Binaries.
3. Copy the needed Telerik assemblies to that folder. Make sure you are copying the Silverlight assemblies, not the WPF ones.
4. Add references to the new assemblies.
5. Clean the solution.
6. Rebuild the solution.
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