I just checked out the site and looked at the event viewer. I see there's also an error thrown prior to the one you posted above that it Could not load type 'Admin/Security/Logoff.aspx' which I think is what is generating the other error.

I have an idea that might work but I'm not 100% sure if it'll fix the problem. There were major changes done to the Authenication portion of DNN when DNN 4.6.0 was released. Prior to that the majority of the authentication process were stored in Admin/Signin. When DNN 4.6.0 was released everything was separated and the providers (including the DNN login control) were moved to DesktopModules/AuthenticationServices and the wrapper for those controls were moved under Admin/Security. What I think may have happened is that you've got some .dll files that weren't removed or updated properly when you did the upgrade and it's causing the error.

Do you have the ability to create a duplicate of your site on your local computer? I'd prefer if you tried this on an offline site rather than your production site in case it all goes the heck. Once you get your site setup under an offline install I'd like you to do a directory compare between the offline site and the upgrade/install version of 4.8.1 that you downloaded. There's a program called WinMerge that works great for comparing files and directories. Remove the .dlls from your offline install that are no longer in the upgrade/install version (except for 3rd party modules your installed). Then give it a try and see what happens when you log out.

Let me know if I haven't made any sense with this post and I'll try to clarify.

Hi,

The logout problem is resolved.  In the dnn database, ModuleControls table the page value for Logout was set wrongly and when I set it to the correct page it worked well.

Rgds,

SS