Hi everybody! First of all, thank you for your answers.
I will try to answers your questions:
@Sepone Yes, the pushback point is correctly set.
@Stu My taxi wides are normally 5 tiles wide.
@EG0611 Yes, it also happened after I saved the game, with a different name (as I have seen in other posts).
@dewitjur probably you are right, but it was not the case, as planes stayed in the same position for many in-game hours, until I had to cancel the turnaround process, in order to allow other planes to arrive.
I believed the problem was in any process related to the stair trucks. I have solved the problem, at least partially, by upgrading the stands with a jetway. After I did this, the planes completed the turnaround process without problems… Could these two things be related @Olof?