I would like to inform you that the support for RadEditor for MOSS is discontinued as discussed in the following blog post: RadEditor for MOSS (SharePoint 2007) Discontinued as of Q2 2012
Version 4.5.1 is also based on RadEditor Classic which development is stopped in 2009 and it does not support the modern browsers. The only suggestion that I could provide is to test the latest official build of the free RadEditor for MOSS 4.5.6
and see whether its link manager behaves better.
Currently, Telerik supports only RadEditor for SharePoint 2010 and RadEditor for SharePoint 2013, which are based on the latest version of RadEditor for ASP.NET AJAX Q1 2013.
the Telerik team