The default ClientIDMode of an asp.net 4+ app is "Predictable". However, the UniqueID is not generated with the "uniquification" of the ClientID. Therefore going from UniqueID->ClientID (which is done frequently in the ScriptResource file) is not possible. The function "_uniqueIDToClientID" by definition is impossible to implement. This results in bugs manifest in several places of the ASP.NET UI framework.
As a workaround I have set the clientIDMode in the web.config to "AutoID", and this works.