Exchange 2010 OAB Password Prompts

Just recently some of our users reported a password prompt box in Outlook repeatedly coming up. After testing I determined this was only occurring while inside our network (Outlook over HTTP), not while connected through OA. We are on Exchange 2010 with mixed Outlook 2007/2010 clients on Windows 7. This issue is occurring with less than 10 percent of our 750 users, and I can't find a common attribute between them that others in the org don't share. If we login to a computer with an affected account on a clean Windows 7 install, the issue persists. Our Exchange install has been up and running for many years without this issue and there were no changes/updates made recently. It seems like the prompt comes up when Outlook attempts to connect to OAB. I've checked OAB file/folder permissions and all are correct according to MS guidelines. I've rebuilt OAB and restarted the server. Can someone please point me in which direction I should take next for troubleshooting?
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HumanScaleDevAuthor Commented:
I changed the internal OAB directory to HTTPS rather than HTTP and tested and no longer get the popup boxes. IIS is not requiring SSL for the OAB virtual directly though. We did make some IE changes a few days ago through GPO. Are there settings in IE that would dictate how Outlook directed http traffic? It's almost as if it's being forced to use SSL for the connection.

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