Is it permissible to marry a woman in one specific situation, yet forbidden to marry the same woman in another situation?

Yes, there are cases in which marriage is forbidden, including: the marriage of a woman in her 'iddah (waiting period) to someone other than her husband, to prevent the mixing of lineages; the marriage of an adulteress until she repents and her 'iddah ends; a man marrying his thrice-divorced wife until she marries someone else and he consummates the marriage with her, and then divorces her; the marriage of a woman in a state of ihram until she completes her ihram; combining in marriage two sisters, or a woman and her paternal aunt, or a woman and her maternal aunt, due to cutting off kinship ties; and combining more than four wives.