That's a relatively common issue for my SQL server database. Every second time when I try to open this database in Outlook, it just doesn't work! Why? I still don't know. I have wanted to fix it so many times, and I still cannot find a proper solution for this very issue. For f sake, I had a couple of customers that actually refused the order as they weren't able to open that damn database in freaking Outlook! Since then, I have always tried to keep a copy database sql server in any other format on any other device.