In our framework, which is build with the Telerik AJAX components we god a issue from a costumer the other day. The RadComboBox did't  work any more.
After a lot of research I discovered the the client was running IE 10 in compatibility mode and indeed I can now replicate the error here at my machine. There are even more controls that are not working as expected in IE 10, in IE 9 and IE 8. For now I have discovered that the RadPanelBar don't open or collapse.
I have also seen that after a click on a button the RadComboBox was working again.

Have any one a suggestion where I have to look for this problem?

Anders Pedersen 

Hello Anders,

If the RadComboBox is not opening a dropdown menu on click this could be caused by a JavaScript error. Please check the browser console and ensure that there are no errors on the page. After the JavaScript errors are resolved the RadComboBox should expand as expected.

If the issue persists would you share your full markup with the related code-behind so everyone that would like to help could have better understanding of your scenario?

Regards,
Viktor Tachev
Telerik