Unable to cast object of type 'System.Web.Security.FormsIdentity' to type Custom While File upload

I am unable to upload the file which return the 500 error. As checked while upload the file the custom authentication changed as form authentication. So our system not allow to upload the file in the form authentication 

Error Message :  Unable to cast object of type 'System.Web.Security.FormsIdentity' to type Custom

Please help

Thanks,

Gautham M