
Oracle acknowledged this bug in Document ID 2338899.1
P6 Professional Patch Set 17.12.1.0 fixes the problem. But users who are not familiar with how to apply a patch can apply a very easy fix. Simply open the project before trying to copy the project. Indeed, both the patch and Version 18 address this bug by opening the project automatically during the copy and paste operation. In effect, users can only copy and paste open projects when using the SQLite database.
Personally, I like to use the SQLite database because it requires no maintenance and can easily be moved to another computer. Moving programs from one computer to another is normally a joyless exercise but not with SQLite. And yes, the database is a program, just like Primavera P6.
I initially resisted using the SQLite database because Oracle had disabled Schedule Comparison (i.e. Claim Digger) when using this standalone database. But eventually Oracle figured out a way to make Schedule Comparison work with SQLite. For anyone who does not need to share a database with other users I highly recommend SQLite.