App Engine tasks that target the backend get 503, even if the backend is running

Many tasks are not completed because they get 503 from the target backend. This is very strange because most tasks are completed, but it seems that if he gets 503, then he will always get 503.

I try to start the tasks manually, and the backend is active and maintained, but if the task was unsuccessful with 503, it will work again with 503 in place.

Any clue?