Hi Roberto Wenzel,
Other customers have reported similar issues with any controls when used within applications with localized resources, if in the resource file appear any resource strings that are empty (""). Please review the you resource files and remove any empty strings. If those resource entries are essential to your app, you can consider replacing the values with space (" ") instead empty string.
This problem is related to Visual Studio, in localized applications, and appears for any custom controls that have string properties, that have empty string value, which is not defined as its default value.
For example, the following control would generate the same error if it is added to your form:
Let me know if can help with anything else.
Regards,
Mike
the Telerik team