Hi Matthias,
Thanks for your interest in syncfusion products.
We suggest you to assign the function name for the client side events to be evaluated as shown in the below code. When declaring code as inline please remove the ";" in the code "objSyncNode.ClientOnItemSelect = "alert('test');", so that it could be evaluated.
[CS]
objSyncNode.ClientOnItemSelect = "test()";
[JavaScript]
Also I have created simple sample and attached in the below link,
treeview1237725155.zipPlease try this and let us know if you have any queries.
Regards,
Vignesh