ActiveSync Freezes with Internet setting applied

Pulling my hair out with this one. I'm trying to support a number of
users with different PDA's and different OS versions (not company
issue).

Just recently they have been unable to sync with their Outlook 2003
clients (we don't sync to exchange). Active Sync versions vary but I
am working on 4.5 as this is the most recent. It will work up until
the point a certain group policy is applied. This policy sets the
value in the 'use automatic configuration script' and ticks the box,
'automatically detect settings' is also applied. All this is under the
Connections - Lan settings of IE.

One port (none of the listed Active Sync ports) is set in this script
and routes internet traffic through our web filter and ISA 2004.

Untick this and it all works again, any ideas??

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