Each version of RadEditor for SharePoint is released and works with a specific version of Telerik.Web.UI.dll. You can find information in the attached readme.txt file. This means that you cannot use 5.3.2 with Telerik.Web.UI.dll version different from 2008.3.1314 without risking to break the control or some of it features.
Note that the development of RadEditor for MOSS is discontinued and it does not officially support version 2012.3.1205.35 of RadEditor for ASP.NET AJAX. You can see this blog post for more information: RadEditor for MOSS (SharePoint 2007) Discontinued as of Q2 2012
My suggestion is to upgrade to the latest official build of RadEditor for MOSS 5.8.16
which comes with Q2 2012 of RadControls for ASP.NET AJAX.
All the best,
the Telerik team
Consider using RadControls for ASP.NET AJAX (built on top of the ASP.NET AJAX framework) as a replacement for the Telerik ASP.NET Classic controls, See the product support lifecycle here.