This is a known problem with the current version of the content editor Web Part. We have already fixed it for the next release. You have two choices if you want to fix this problem. You can either switch to the built-in editor dialog for links, or you can open a formal support ticket and request that we send you a RC build of the upcoming editor.
To enable the alternative links dialog, open the editor's tools file (ToolsFile.xml) from \Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\18.104.22.168__1f131a624888eeed\RadControls\Editor\ and replace the tool MOSSLinkManager
with just LinkManager
the Telerik team