Outlook 2007 fails downloading OAB every morning

Hi

I have a strange problem. Every morning when people starts Outlook 2007 it logs 0x801901B8.

The setup is Exchange 2010 with a CAS array. I only use OAB web distribution.

Here comes the strange part :-)

I I make a manual sync of the OAB it Works.
If I do a "Test E-mail autoconfiguration" everythings seems ok.
If I try to connect to http://casarray/oab/"oabguid"/oab.xlm everything is okay.

Why does the Outlook clients fail on the first sync every morning?

What can I try?

BR
Steen