Client Certificates - "Page cannot be displayed" error

Hi there,

I've got a curious problem regarding client certificates. I'm using Windows

2003 Server Certificate Authority client certificates for a web application

to limit access to a web application. I've also created the client

certificates from the standard 'User' certificate template.

The problem I've got is that for a while all users could use the app fine

with these certificates in place, however over the past month or two only

some users report that they can't get into the web application and they get a

"Page Cannot Be Displayed" error message (client machines are Windows XP

tablet edition running IE 6 SP2). I've checked their machines and the

certificates are still there and haven't expired (nor have they been

revoked). If I remove the certificate then I get the expected "Page requires

a client certificate" error message. If I then create another client

certificate on the machine it then works fine, but after a random period of

time it stops working again. Every user is allocated their own certificate

and out of 20 users only the same 5 or 6 people seem to be having this issue.

I found this Microsoft Knowledgebase article and thought that this may help...

http://support.microsoft.com/kb/811383

However it says that the issue occurs on IE 6 SP1, plus I've also upgraded

some of these machines to IE 7 and its still happening.

I'm not really sure where to start looking for a solution so any suggestions

would be appreciated.

Thanks.