Hi Inma
Am I right to assume that these essentially translate into English as the same thing? Otherwise, could you explain any other differences in meaning that might exist?
1) Su avión ha debido retrasarse
2) Su avión debe haberse retrasado
AND that adding 'de' also makes no difference to the meaning (other than maybe 'strengthening' the assumption as we learned about here: Spanish modal verb Deber versus Deber de (obligation and assumption))?
Saludos