Postbacks (usually on datagrid sort) are giving client 'Cannot find server'

[Kuidros via DotNetMonster.com]

Greetings,

A quick summary of the problem:
A user logs in, selects criteria from dropdownlists, which postback
successfuly, then 'Submit's the search. The datagrid is rendered correctly,
with no issues. However, when the client attempts to click a column header
to sort, or if after the initial rendering, resubmits, the page returns with
a 'Cannot find server or DNS error' page. This does not happen on my system,
nor any systems I've had the opportunity to test it on.

Any suggestions what client settings may be bringing this error up? (Also,
this happens on all browsers, and FireFox returns Document Empty).

Thanks
Kuidros