DNN 4.3.5 - I'm running into the same error.  I have a placeholder in a module that I am building the content for.  When it does the AJAX call, I get the same type of error.  I have tried various levels of this.Parent, but nothing seems to work.  To further confound me, the ClientID changes depending on what other modules are loaded.

Instance 1:
2 Modules Loaded on Page - both making use of the ClientAPI - both work fine.  The ClientID for the 2nd module on the page is:  dnn.xmlhttp.doCallBack('View dnn_ctr3288_View'...

Instance 2:
1st Module on page is hidden due to security (same applies to removal of 1st Module from page).  The ClientID for the 2nd module on the page is: dnn.xmlhttp.doCallBack('_ctl0 dnn_ctr3288_View__ctl0'....

The first module works great by itself (it is not loading into a placeholder).  The second module throws the cast error when by itself.  Both are implementing ClientAPICallbackEventHandler.  Any suggestions?  Why would the ClientID change?

I am dynamically building the content of the parent control (DNN Module) during the Init proceedure, so all controls should be there.  I am not trying to handle the RaiseClientAPICallbackEvent in another control, but in the Module.  If the module is the only one on the page, then its ClientID shouldn't change on post back and thus shouldn't be the cause of the error.

When calling the ClientAPI with ClientAPI.GetCallbackEventReference(this, "payload", ....), "this" should reference the module, correct?

A little confused here.   When a page is viewed by someone with different permissions I would assume that the page would render differently than if there was no permissions.  By rendering different, I mean different controls are used.  When different controls are shown, the possibility exists that different IDs are assigned by ASP.NET. 

The confusing part is why should this matter?  As long as the permissions are the same between the time the page was first rendered (registered the callback) and the time of the callback, everything should render the same and thus the control should be found.

I am not sure what more I can say, I believe I answered your question, but lack the needed information to discern where your issue is.