Currently I use nightly 4775.
I am not sure if it is bug or not - when planes do it on only one airport. Airplanes will approach to end of taxiway and there they stay - and say that they are waiting for free runway - but it is free.
And after action for re-setting (forcing) plane's next station, it stay on runway's end - and still wait for free runway. And after next forcing of its next station they take off and fly to target airport - and after return to this airport planes' behaviour is the same.
I really don't understand this. For this reason I decided to call this in Help requests first - with possibility of moving into Bug reports.
Maybe the reservation of an airplanes failed. YOu can click on such tiles with the block clear tool ('b') to free the reservation. Or try saving and reloading.
You was right - last two tiles of runway for taking off were blocked. So I am not sure how it happened.
I solved it with rebuilding of that runway.