This seems more like a problem with your network than with DOMPDF. Are
you accessing DOMPDF through a proxy, or is DOMPDF accessing a remote
resource through a proxy.
I'd also like to note that, depending on the complexity of your
document and the available system resources, DOMPDF may have problems
generating a 600 page PDF. Is PDF generation successful with something
less intensive, like a single-page document?