Can I spend a year without registering?

According to Article 17.1 of Regulatory Agreement 13084, of 10th April 2019 (FOU 481):

'In the event of not registering during the relevant period for academic tutoring, doctorands may not register on the same PhD programme for the following two years. In any event, the academic commission for the relevant PhD programme must approve admission and the same academic file shall be maintained. This readmission may only be granted once.'

For further information, please see the PhD readmission section in the Doctoral School Procedures section.