Trouble syncing when using a "pac" file for auto proxy configurati

As stated we are having problems syncing Windows Mobile 5 clients when on the
pc (XP SP2) a proxy autoconfiguration script is used (proxy.pac file). Active
sync just hangs and we get the following error in the WSESMgr.log file:
ERROR in DetectAutoProxyWinHTTP : hr = 0x80072f94

The version of ActiveSync is 4.5 (but we've tried all 4.x versions..). Also
it's not just one machine having this problem but all pc's that we'd like to
sync with a mobile 5 pocket pc.

When we manually configure the proxy on the pc ActiveSync works fine. We
sync with the local Outlook client and not the Exchange server. Besides that
we do not use advanced networking for the ActiveSync connection. (Using
advanced also does not make a difference).

Not using a proxy.pac file is not desirable as the users would then have to
disable the proxy server for internet browsing when not in the office.

The problem does not seem to be in the proxy.pac file. A completely empty
pac file (Return "Direct" also does not solve the problem. Only manually
entering the proxy server in IE or not using a proxy server or pac file at
all works.

Could anyone offer some advice on this issue?