I have the same problems with the 64bit version.
The copy command is working well until I start a second instance of PL/SQL Developer.
I mentioned this on the beta forum as well.
Hope you will find the bug.
When I start just one instance of PL/SQL Developer copying a marked string to the clipboard is working well.
If I start the second instance the entire application is hanging for a little second after a I copied.
It is not a matter of the instance where I try to copying content to the clipboard...