Gemara

(Mishnah): If one was Mekadesh a woman with any of the following, she is not Mekudeshes: Orlah, Kilayim in a vineyard...

Pesachim 21b (Mishnah): After the (permitted) time on Erev Pesach expires, Chametz is Asur b'Hana'ah.

The Chidush is, it becomes Asur b'Hana'ah mid'Rabanan (while it is still permitted mid'Oraisa):

(Rav Gidal): If one was Mekadesh with wheat of Kordenisa from six hours or later, we are not concerned for the Kidushin.

Rishonim

Rif (Pesachim 3a): If one was Mekadesh with wheat of Kordenisa from six hours or later, we are not concerned for the Kidushin.

Rebuttal (Ba'al ha'Ma'or): Rav holds like R. Yehudah, who forbids Hana'ah from Chametz from midday on Erev Pesach. Even though the Stam Mishnah is like him, the Halachah follows R. Shimon, who forbids only mid'Rabanan. Therefore, we are concerned for Kidushin even in the seventh or eighth hour.

Defense (Milchamos Hash-m): R. Yehudah holds that we are not concerned for Kidushin with Chametz mid'Rabanan. Likewise, R. Shimon is not concerned when Chametz is mid'Rabanan. However, we are concerned during the sixth hour even for Chametz mid'Oraisa, for people are burning it and do not realize that it is Asur b'Hana'ah. Perhaps they will confuse this with the fourth or fifth hour, and permit an Eshes Ish!

Ran (DH ha'Mekadesh): Even R. Shimon agrees that Chametz is forbidden mid'Oraisa after six hours.

Rosh (Pesachim 2:2 and Ran ibid.): Rashi explains that 'from six hours and onwards' is from the start of the sixth hour. Even though the wheat is hard (and does not become Chametz easily), if water came on it there is no Kidushin. The words connote after six hours; we say that the Torah forbids from six hours and onwards, i.e. from midday. R. Tam explains that wheat of Kordenisa is Chametz Nuksheh (not proper Chametz). It is forbidden only mid'Rabanan.

Rosh: Even though we learn Chametz Nuksheh from a verse, it is mid'Oraisa only from the first day of Pesach. On Erev Pesach it is forbidden only mid'Rabanan.

Ran (ibid.): Perhaps we are concerned for Kidushin with Chametz mid'Rabanan at a time when Chametz is forbidden mid'Rabanan. However, the Ramban is concerned for Kidushin with Chametz mid'Oraisa from the start of six hours. People shy away from it after six hours, but during the sixth hour they are engaged in burning it. If we would ignore Kidushin during the sixth hour, perhaps they would ignore Kidushin earlier in the day.

Rambam (Hilchos Ishus 5:1): If one was Mekadesh a woman with something Asur b'Hana'ah, e.g. Chametz during Pesach, meat and milk, she is not Mekudeshes. This is even if it is Asur b'Hana'ah mid'Rabanan, e.g. Chametz during the sixth hour on Erev Pesach.

Magid Mishneh: Even when Chametz is Asur mid'Rabanan, e.g. during the sixth hour, she is not Mekudeshes. The same applies to other things. The Meforshim resolved this. Also the Ramban agreed that she is not Mekudeshes with any Isurei Hana'ah.

Lechem Mishneh: The Ramban holds that she is Mekudeshes in the sixth hour. The Beis Yosef (DH u'Mah she'Chosav she'Afilu, citing the Ran) says that the Rambam holds that even then she is not Mekudeshes! If so, the Magid Mishneh means that the Ramban agrees about other Isurei Hana'ah. If the Magid Mishneh holds that the Rambam agrees that she is Mekudeshes during the sixth hour with Chametz mid'Rabanan, the Ramban truly agrees to what the Rambam said here.

Rosh (Kidushin 2:31): We are concerned for Chametz mid'Rabanan, e.g. wheat of Kordenisa, at a time when Chametz is forbidden only mid'Rabanan (the sixth hour on Erev Pesach).

Poskim

Shulchan Aruch (EH 28:21): If he was Mekadesh with Chametz mid'Oraisa at a time when it is forbidden mid'Rabanan, or Chametz mid'Rabanan at a time when real Chametz is Asur mid'Oraisa, she is Safek Mekudeshes.

Question (Chelkas Mechokek 38,39): The Ramban is concerned for Chametz mid'Oraisa at a time mid'Rabanan. Why is the Shulchan Aruch concerned for Chametz mid'Rabanan at a time mid'Oraisa? If he is concerned for the Ba'al ha'Ma'or's opinion, he should be concerned even for Chametz mid'Oraisa after six hours! The Beis Yosef (Siman 443) said that one who rules leniently like the Ba'al ha'Ma'or should be reprimanded.

Answer (Beis Shmuel 53): The Shulchan Aruch is concerned for the Ba'al ha'Ma'or's opinion. We hold like Rav Gidal that we are not concerned for Kidushin at a time mid'Rabanan or if the Chametz is mid'Rabanan. Rav Gidal forbids Chametz mid'Oraisa after six hours, but the Ba'al ha'Ma'or rules like R. Shimon who forbids only mid'Rabanan. Therefore, we are concerned for Chametz mid'Rabanan after six hours, for both the time and Chametz are mid'Rabanan. He agrees that we are not concerned for Kidushin with Chametz mid'Oraisa (even during the sixth hour)!

Gra (57): The Shulchan Aruch rules like the Rambam. If so, He should not be concerned for Kidushin at a time mid'Oraisa (like the Ramban is), and surely not when the time is mid'Oraisa, for everyone argues with the Ba'al ha'Ma'or.

Shulchan Aruch (ibid.): If he was Mekadesh with Isur Hana'ah that is totally mid'Oraisa, e.g. Chametz mid'Oraisa at a time mid'Oraisa, she is not Mekudeshes.

Bach (DH Kosvu): One answer in Tosfos (Pesachim 29b DH Ein) is concerned for Kidushin if the ashes of the Chametz are worth a Perutah, for we hold that Chametz must be burned, so the ashes are permitted. We hold that Davar ha'Gorem l'Mamon (something without intrinsic value, but still has value for someone) is like money. If so, we must always be concerned for Kidushin, lest the ashes are worth a Perutah in Madai! Rav Gidal was not concerned, for he holds that Chametz need not be burned (so the ashes are forbidden). Tosfos (Kidushin 56b) is unsure about Kidushin with Orlah that is worth a Perutah for abnormal benefit. Therefore, we should require a Get.

Chelkas Mechokek (39): Even though the Shulchan Aruch (OC 445:1) says that Chametz can be destroyed in any way, that is a stringency (if one cannot burn it, he must destroy it another way). Perhaps regarding Kidushin, we are concerned lest it must be burned. However, if she does not know that it is Asur b'Hana'ah, surely she is not Mekudeshes.