After deleting site and importing content, and trying to save it got a message:
“Received Content from server differs from what you have. Would you like to load changes from server?”
Haven’t found any clues in documentation, why can I get it?
Is this in the content wizard? If so this happens if state between editor and server breaks, or there might be a bug.
Yes, it’s in content wizard. How can state between server and editor be broken?
The obvious answer is that somebody else have changed the content on the server after you fetched it to your editor.
If that is not the case, then there are some possible explanations if there are problems with the network, especially if you run on a cluster, but most likely it is then a bug. I see you are using version 6.6.0. You should upgrade to the latest 6.7.3, or wait one more week for 6.8.0. There are a couple bugs of this type that have been fixed since 6.6.0.