I've got a strange problem with an Exchange 2007 server running on SBS 2008. It's been working fine for users for the last 5 months, then 5 days ago they started getting a problem where Outlook would popup a window asking for credentials. If the user enters there credentials it pops up again and again, however if the user cancels the box it goes away and outlook continues to work fine. These clients all connect using Outlook Anywhere, and OWA access is also working fine.

I had thought this message box may have been to do with other POP or IMAP accounts being configured in Outlook, but it's not the case. I have re-configured Outlook Anywhere on the server but to no avail. Using the RPC diagnostic tool in Outlook shows no problems.

There are no errors showing in the event log. The server seems to be running fine, so to me this seems like either a client, or an RPC over HTTP issue, but I'm not getting anywhere finding the cause, any help is appreciated!