IE8: Can't open component properties box in Template Composer

See screenshot.

To reproduce:

• Browse to the Go Green test server using Internet Explorer 8
• Open the channel manager, and click a channel
• Click "Edit template"
• Click a component

Expected behavior: The properties box for the compnent opens
Actual behavior: An error occurs, and then the page is reloaded: "Error occurred handling message from the iframe. The site gets reloaded to prevent inconsistencies."