Hi Sascha,
I apologize for the delay in responding.
I am afraid that I was not able to reproduce the issue. Please refer to the following steps that I tried to reproduce it.
1. Drag-n-dropping the callbackpanel-control
2. syncfusion.shared.web will also be added
3. doubleclick in the properties of a callbackpanel to the CallbackRefresh-event
4. But No Error.
Here is the video clip for your reference:
http://www.syncfusion.com/Support/user/uploads/VideoClip_d91bf6b4.zipPlease have a look at the above Video clip and let me know if I am missing some thing. And also, kindly Re-initialize Visual Studio.NET Toolbox and then create the application and let us know if the problem still exists.
Thanks,
Melba