I compile a .drl file and save it as object in .drl.compiled file.
Loading that file does not work if I use insert() or retract() facts that are of a type declared in the .drl file.
The way to circumvent this is to use the full class name in the insert, like this:

insert(new vcm.planner.lock.standard.PossibleSlotLock($ship, $chamber,
$side, $schedule, $eta, $schedule.getTick(), "existing"));

And cast the variable to the full class name, like this:

rule "clean up possible slot lock, when no longer to schedule"
when
$p : PossibleSlotLock( $ship; )
not ToScheduleLock( mover == $ship )
then
retract( (vcm.planner.lock.standard.PossibleSlotLock)$p );
end

This did work in 5.4.0.Final.