Notes/Domino 6 and 7 Forum : Single Logon - the "Login to Notes using your operating system login" checkbox is greyed out

I am currently involved in rolling out Notes and Windows 2000 to replace GroupWise and Netware at a large company. This company has a number of offices in Europe including a few here in the UK.

Each site has its own Domino server replicating to head office and each site has it's own branch in the active directory.

In one of these offices, which appears in Active Directory in
Company>EU>UK>London>Regent Street

...there is a Domino server RegentStreet/Company

So, we're over there today installing W2K Pro and ND6.03 on their machines. Notes installs fine, and I can confirm that the Single Client Logon service is installed and running. But when you start Notes it asks for your password, when you go to Security, the "Login to Notes using your operating system login" checkbox is greyed out.

The reason I've explained the active directory structure and the fact that each site has its own Domino server is because this problem is ONLY happening at Regent Street for Regent Street users and I can't replicate it at any other site. Therefore, I can't work out what the problem is!

If someone could tell me what would cause the checkbox to grey out, I'd be very grateful.

Thanks

Steve