Try this thread for some solutions to this problem:

http://dotnetnuke.com/Community/ForumsDotNetNuke/tabid/795/forumid/108/threadid/57742/threadpage/3/scope/posts/Default.aspx

If you are using blowery compression there is a config section inside your web.config.

In the exclude paths you need to have one that looks like this

Try excluding the admin controls from using compression with a node in the blowery config section like this:

    <blowery.web>
       <httpCompress preferredAlgorithm="gzip" compressionLevel="high">
          <excludedMimeTypes>
             <add type="image/png" />
             <add type="image/jpeg" />
             <add type="image/gif" />
             <add type="application/zip" />
             <add type="application/x-zip-compressed" />
             <add type="application/x-gzip-compressed" />
             <add type="application/x-compressed" />
             <add type="application/octet-stream" />
             <add type="application/pdf" />
          </excludedMimeTypes>
          <excludedPaths>
             <add path="ebresource.axd"/>
             <add path="install/" />
             <add path="admin/" />
          </excludedPaths>
       </httpCompress>
    </blowery.web>