ASP Behavior Changes On Different Client Machine

I know this is actually impossible, but didn't know how else to describe it:
I have created a Content Management System for a client using ASP connecting
to a Access database. It doesn't do any fancy things: INSERT, UPDATE, etc. I
also use ASPUpload to upload images onto the server.

When I test my CMS from my own machine and others, it all works fine.
However, testing it from my client's machine, the Updating or Inserting of
data ends up returning a 'page not available' error (the asp page calls
itself when the form is submitted) and the changes to the database are not
being executed.

I really can't figure out why this is happening. Could it be a setting on
the client's browser?