I say, and with God's success: It is not considered an excuse, and if the person in I'tikaf becomes ill during a mandatory I'tikaf and breaks their fast for a day due to the illness, they must repeat the I'tikaf whether the illness is severe or not; because fasting is a condition of I'tikaf and it has been missed, and worship does not remain without its conditions just as it does not remain without its pillars. Refer to: Al-Mabsut 3: 118, and God knows best.