Unable to copy large scenario

I tested copying a sample, a small scenario - copying works. I tried a larger one - it was copying for an hour (or longer_ and never finished. I tried exporting the scenario to re-import. It kind of showed that the import was successful but the re-imported scenario never showed up on my list of scenarios (I have all of them on one page).

This is a major problem for me, because I need to split the project for the client.