Thanks Martin,
As you sent the mail regarding this, I would like to share your quotes.
The reason is that because of some API Changes in Kentico 8, CMSContext doesn't exist anymore.
Method
CMSContext.SetCurrentUser(CMS.CMSHelper.CurrentUserInfo)
was moved to
CMS.Membership.AuthenticationHelper.SetCurrentUser()
For more details regarding API changes in Kentico 8, please read this article:
https://devnet.kentico.com/articles/api-changes-in-kentico-8
Thanks