I can debug procedures/packages belonging to another user when logged on as SYS/SYSTEM, but can not debug procedures/packages belonging to SYS/SYSTEM.
In version 9.1 the ability to step through and debug SYS/SYSTEM objects was removed for security reasons.
In version 9.0.1 and older there is a setting that will allow you to step through and debug SYS/SYSTEM objects.
WORKAROUND for version 9.1:
WORKAROUND for version 9.0.1 and older:
Make sure View | Options | Debugger | Step through sys/system Procs is enabled.
For version 9.1, you should be able to debug and pick up errors in the code. But you should not be able to step through, like apply breaks or pick up watches, with SYS/SYSTEM objects, regardless of log in. You should be able to step through and debug objects that belong to other schemas while logged in as a SYS/SYSTEM user.