Selenium Server error: Unable to get browser

I'm running Selenium Standalone Server 2.25 on Windows 7, with Internet Explorer 9 as a browser, and I'm getting this error for every test that requires opening the browser:

Selenium::WebDriver::Error::UnknownError:
       Unable to get browser (WARNING: The server did not provide any stacktrace information)
       Command duration or timeout: 10 milliseconds
       Build info: version: '2.25.0', revision: '17482', time: '2012-07-18 21:08:56'
       System info: os.name: 'Windows 7', os.arch: 'x86', os.version: '6.1', java.version: '1.7.0_07'
       Driver info: driver.version: EventFiringWebDriver
       Session ID: 356e9fc8-2d2b-4808-964c-4a1fea7415f9 (org.openqa.selenium.WebDriverException)

What am I doing wrong? The exact same information appears on the Selenium Server console.

Solution

The error disappeared after enabling the Protected Mode settings for all the zones in Internet Explorer (Tools > Internet Options > Security tab).