Does anyone know why the HTML editor (RadEditor) pop up for setting a hyperlink works only 2 in 10 times?

the other 8 times I get the following error:

HTTP Error 404.0 - Not Found

for the resource:

/DesktopModules/Admin/RadEditorProvider/DialogHandler.aspx

More specifically below are the URsL when it does and doesn't work.

Can anybody spot the difference? The difference in length is only 5 characters so I don't think it is that. Working is 1440 chars and broken is 1445 characters.

Working URL:

http://www.ukperformingarts.co.uk/DesktopModules/A...

Broken URL

http://www.ukperformingarts.co.uk/DesktopModules/A...

Thanks in advance,

L