Created Unassigned: CKEditor error "You are Not Authorized!" after attempting to upload image [7960]

When using the CKEditor, I am getting a "You are not Authorized!" popup after trying to upload an image.

To recreate this error:
1. Edit an HTML module.
2. Click on Insert Image
3. Click on Browse Server
4. Browse to the folder to upload the image
5. Click Upload New File
6. Click Add file(s)...
7. Select file, click Open
8. Click on Start upload(s)
--Window closes, you are back to the Browser-Modus: Image window
--No image was uploaded
--Click on any other folder
--Popup: "You are Not Authorized!" Click OK
--You are back to the Image Properties window, click Cancel
--You are back to the HTML editor window, click Cancel
--You are back to the page and are now LOGGED OUT

__It appears that somewhere in the process the user gets logged out.__

The user that this has happened to has Edit rights to the page and permission to browse, open and write to the folders that the image was being uploaded to. The user can upload this image in File Management without a problem.

The file that is being uploaded is 20KB.

I have tested this with the admin user and it worked fine without a problem.

The Log Viewer shows:
DotNetNuke.Services.Exceptions.PageLoadException: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

I tested in IE, FF and Chrome with the same results.

I switched back to RadEditor and did not have this issue.

DNN 07.03.04
CKEditor Manager 2.1.0

Thanks for your help!