Outlook 2007 losing shared calendar permissions

Hi All,

My users are on Outlook 2007 and we connect to an exchange server (2003). The problem we are having is that previously shared calendars are suddenly becoming unavailable. The message that pops up is:

'The set of folders cannot be opened.'

A more detailed message suggests the following:

'This error usually occurs if the OST or PST file you are using is:
> Unavailable
> Protected with file permissions.
> On a share on a server and the network is down.
> Corrupt

To correct this problem, store the OST or PST file on the local computer.

You may also want to run the scanost.exe and scanpst.exe tools to verify file integrity.'

I have tried scanning them and they are fine. When I look at the permissions they are gone. When the user re-shares the calendar it is ok again but often looses the permission after a re-boot (but not always).

Any ideas welcome...