To check the control I used the snippet you supplied and this worked ok but this was within a seperate web applicaiton to where we are having the issue.
to give some more detials on the issue;
We are using the control within a SharePoint 2010 webpart. The control works 99% (once or twice we have had the error) of the time on mine(windows7 ie8) and Mats(windows7 ie9) machine .
On machines which are Windows XP and IE7 we get the stated error on every attempt.
Also firefox on my machine gives the following error:
The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again.
Are there differences between how the browsers and/or the OS handle this type of authentication..?
My next step is to create a sharepoint 2010 webpart with just the cutesoft control on and see if this error occurs as soon as its ran within a SharePoint Enviroment.
Any information on why it would work on some set ups and not others would be very