I say, and with Allah's success: The I'tikaf of a woman is valid with the husband's permission if she has a husband; because she is among those who are capable of worship, and the only hindrance is the right of the husband. When permission is granted, the hindrance is removed. If a woman vows to perform I'tikaf, her husband has the right to prevent her. If she is divorced, she must fulfill her vow; because the husband has ownership of the benefit from her, and in I'tikaf, he is delaying his right to benefit from her. Therefore, he has the right to prevent her as long as she is under his ownership. If she is divorced from him, she is obligated to fulfill her vow. This is because the vow is valid from her as she is among those who are capable, but she was prevented from it due to the husband's right. When his right is removed by divorce, the hindrance is lifted, and she is obligated to fulfill her vow. Refer to: Al-Badai 2: 109, and Allah knows best.