Only One User Cant "Add To Cart" why

Nop working fine for all clients from 3months. every user puchase goods perfectly
but from today all user work fine but one specefic user do work with our site very well
dut when he click on add to card button or click on shopping card icon there will be a
Error msg shows like

{We're sorry, an internal error occurred that prevents the request to complete.
Our supporting staff has been notified with this error and will address this issue shortly. We profusely apologize for the inconvenience and for any damage this may cause. You might want to try the same action at later time. }

pls help me its very urgent because this client is very valuable for me.

on editing my web.config  [change RemoteOnly to Off] Error display this

A generic error occurred in GDI+.
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Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Runtime.InteropServices.ExternalException: A generic error occurred in GDI+.