We have upgraded the website from 6.1 to 7.1. After deploying it to the server and truing to access pages under Sitefinity backend we see the following error:
You are trying to access item that no longer exists. The most probable reason is that it has been deleted by another user.
There is no such a problem on a local machine running the solution in Visual Studio.
We have also checked app data/sitefinity/logs folder and it is empty.