This could be because the request filtering in more than the allowed size limit.
Check the iis log files to get a request code. It would be something like,
|Status Code||Substatus||sc-win32status(windows system error code)||time-taken|
links below has more info on the substatus message:
if 404.13 is the status then increase the
allowed content size then go to
Request Filtering option in the IIS under the particular site and select
Edit Feature Settings on the right hand pane and increase the value for
Max allowed content length.