Try recovering your cloned (empty) file and see if you can import the data into it. Perhaps your file is damaged in some way.
If you can import successfully into your recovered copy, best practice is to locate an undamaged back up copy and use it.
Thank you for your reply. Recovering the clone and importing into it worked! My only concern now is whether it will be safe to use this new version. Any thoughts on that?
It would be safer to revert to a working backup now if you can rather than using the recovered file.
There may be more things wrong which you are unaware of, these could potentially cause problems in the future, even if they are not obvious now. If this was the case you would have a more time consuming job of having to revert to a much earlier backup, especially if you had made a lot of changes to the file since..
If you can't find a undamaged back up copy to use, you are faced with balancing the risk of using the recovered file vs. the time/cost of rebuilding your file.
You can use your recovered file and watch it carefully for any issues--risking future problems if something was not correctly repaired by the recover operation.
You can also start with a new file and import all your tables, scripts, layouts into this new file. This is tedious, and there is a chance that you will import the problem from the original file into your new file. (I'd import the tables from the recovered copy and the rest of the system from the unrecovered file with frequent saved copies along the way.)
Thank you both for your insights. I think that moving to an earlier, uncorrupted file is still an option (I keep a fairly detailed log of all of my changes).
I just wish I knew what caused the problem in the first place. We have many other, larger databases in use, and the thought of having to rebuild one of THOSE.... well, it would not be pretty.