Did you make sure you have viewState enabled on your site?
Please check also this document: http://world.episerver.com/Documentation/Items/Installation-Instructions/EPiServer-CMO/CMO-2-R2/Configuring-CMO-2-R2-to-support-NET-40/
We are having the very same issue as well. Was this resolved?
We tried the solutions that Sergii provided but it still does not work for us.
NOTE: We have an Episerver Enterprise 6 R2 site with CMO 2 R2 with .NET 4.5
We have the same issue.
Does anyone know how to resolve it?
I've installed EPiServer CMO 2.1 using Deployment Center on EPiServer CMS 6 R2. Clicking Add campaign or Add test buttons in CMO interface, I'm getting the following error: