Marco, I did double-check, "Copy comma separated" doesn't work. But this is only specific to "Foreign key references", it works fine with other subfolders like "Indexes" for example.
the problem still exists.
As said in PL/SQL Developer Forum Topic 004742 you could not "Copy comma separated" when there are more than one entry in "Foreign key references".
If you mark a single entry it works as in all other subfolders of the browser.
Is there an estimate available when the fault will be fixed?