I'm having the same issue.  It seems with Mac clients using either Safari or Firefox they get this error.  Works fine on Windows (IE and FF).  It doesn't matter how many files they try to upload - 1, 10, or 100.   I've looked at both threads that you provided and still not sure what the solution or workaround is.  I've upgraded to the latest version of AjaxUploader.  I'm using the MagicAjax C#, essentially the same code as the example provided.

 

Any solution to this would be appreciated!