This indicates that somebody used the Authorization function as a DBA user, and has created an authorization table. Because it is probably empty now, nobody (except SYS) can use PL/SQL Developer. You will need to start PL/SQL Developer under the SYS account again, go to File > Authorization, and press the "Drop table" button. You can also grant the "All" privilege to "PUBLIC".
For more information, see chapter 22 in the User's Guide.