My company's Exchange 2K10 started prompting all Outlook Clients for auth Tuesday afternoon. If you just drag the prompt to the side and ignore it email works fine. If you input your username and password it prompts again within 3 seconds to 10 minutes. If you hit cancel or close out of the window, Outlook loses connection to our server. It doesn't prompt immediately on startup. It starts connected. So we have everyone moving the window to the edge of their screen and doing nothing and it continues to work fine for almost all our clients. We have a few people using Outlook Anywhere and they cannot use their email at all.
Domain controllers and Exchange have been rebooted more than once, and all services appear to be functioning. There is nothing in the event/error logs. The network connection is secure, cables, routers and switches are all fully functional.
I know this isn't really a programming question but we're all very stumped.