Frameset Refresh - works in development, not production

[Norman]

I have the following setup working in development (local machine -
XPPro/sp2) but it does not work in production (Server 2003/IIS6 over
Internet):

I have a frameset, opened as a modal dialog via showModalDialog(), that
contains two pages (each in own frame).
One shows thumbnails (thumbs.aspx)
Other shows full-sized image (fullsize.aspx), and allows user to upload new
images. When a new image is uploaded, the thumbnails need to be refreshed.

The following JavaScript is executed via RegisterStartupScript from the
fullsize.aspx after user uploads a new photo:
string scriptString = "<script
language=JavaScript>parent.thumbFrame.location.href =
\"thumbs.aspx\"</script>";

Please note that it works exactly as intended - but only on the local
development machine. When I move it all to production, the thumbsFrame does
not reload itself. I have verified that all file versions are the same
between development and production.

Any ideas? I need this to work in production just like in development.

Thanks!