The problem is that ISA Server blocks requests whose query string contains ascii characters with code greater than 128. This is a default http filter from ISA Server called "block high bit characters" (http://technet.microsoft.com/en-us/library/cc302627.aspx
). So, basically I was actually using a localization setting for spanish for the RadEditor as follows:
<telerik:RadEditor ID="Editor" runat="server" BackColor="White"
<ImageManager DeletePaths="~/Images/EmailImages" UploadPaths="~/Images/EmailImages" ViewPaths="~/Images/EmailImages" />
It turns out that the query string for the dialog handler for the ImageManager had ascii codes above 128 as shown in the word "imágenes", specifically the letter á
The solution to this problem was to go to open up the "RadEditor.Tools.es-ES.resx" under the "App_GlobalResources" folder in your solution. Then replace the value"Manejador de Imá
genes" with "Manejador de Ima
genes" for the "ImageManager" string as shown in the attachment. Notice that there is no tilde in the letter a. You may want to do this in the rest of localization resources if you are experiencing requests being blocked by ISA Server.
After this change everything worked because the query string no longer passed ascii characters with codes greater than 128