In case you have recently upgraded the version of controls you are using from a version prior to 2011.3.1020 to Q3 2011 or a newer version, it is possible to observe such behavior. You can refer to this article which explains the differences of the use of RadRichTextBoxRibbonUI and lists some quick steps for upgrading a project.
If this does not help you resolve the issue, please open a support ticket and attach your application, so that we can investigate the problem further.
All the best,
the Telerik team
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