12. Another 1-2-3 instance is using SOLVE123.DLL. Try again later.

This message should appear only if you are running multiple instances of 1-2-3, and you click on Solve while another 1-2-3 instance is also running the Solver. Wait for the other 1-2-3 instance to finish solving and then try again. If this message appears under any other circumstances (most likely due to previous problems with 1-2-3), you should restart Windows and then try again.