Could you clarify in which scenario this problem occurs - SharePoint forms (in Lists, Wikis, Blogs, etc.), Web Part, Web Content Management? In addition to this, please, add your changes and to the ListConfigFile.xml and ListToolsFile.xml.
If you still experience the problem, please, open a support ticket and send us a screenshot of the message, along with your modified files - ConfigFile, ToolsFile etc.
I hope this helps.
the Telerik team