I think I found a problem with the unified editor as substitution variables are not working properly.
I tested this simple query “select &aa from dual” and it ends with an ORA-01008 (not all variables bound).
It took me some time to see the Substitutions panel appear automatically on the right side of the window, but anyway, even specifying a value over there doesn’t work.
And yes, my button (:&) on the toolbar is enabled.
Hope it’s useful,