To clarify, I can use PL/SQL developer on other machines logging in to the same instance that I am prevented from logging in to from the 2 desktops in my office.
I too ran into this today.
I have been a daily user of pl/sql Developer for 8 years and this is the first time it has happened.
It is happening to me on 2 different machines so I would guess it is something with oracle.
I have a session going from yesterday and it is alright. I tried to use it...