This is most likely a javascript bug. Try upgrading to the latest version of Linkr. If that doesn't do the trick, head to the plugin's page in Administrator » Extensions » Plugin Manager » Content - Linkr and set the Compress JS Files parameter (advanced) to "No". If all fails, head to the support page and explain what you were trying to do. Make sure to specify which browser, editor, Joomla version and Linkr version you're using.