From time to time, when I try to update the order status in Orders section of admin, I get the following error message in IE

The website cannot display the page

Most likely causes:
• The website is under maintenance
• The website has a programming error

More Information
This error (HTTP 500 Internal Server Error) means that the website you are visiting had a server problem which prevented the webpage from displaying.
For more information about HTTP errors, see Help.

If I click on browser's Back button, I get back to order page and everything is fine. However, if I refresh the page, I get back to Order section with a red X in customer Notified section.

If I get out of the Order section and get back to it again, the order is disappeared! Even though on main Admin page I see the reference to that order.

What's the problem? Is this related to my DB or my hosting company?