What version are you using? It sounds like you upgraded to CMS 11 but there are some configuration that were not properly updated. That class was moved to EPiServer.Cms.AspNet now
After some research, I got a better error message that led me to the problem. It was a chief2moro imageextension that was incompatible. Thanks for your time.
I'm stuck on this error message when trying to publish a page in EPiServer:
Could not load type 'EPiServer.Core.ThumbnailManager' from assembly 'EPiServer, Version=10.10.4.0
Does anyone know what the problem is?