Is there a connection between marital problems and the spouses reaching a dead end, and their committing the sin of a man having anal intercourse with his wife? And what is the way to deal with a wife whose treatment of her husband has changed and who denies the facts?

If the mentioned error is the husband approaching his wife in her back, then it is a grave abomination that necessitates repentance. It is not permissible for the wife to demand divorce without a legitimate reason, and reconciliation is preferable. If the wife's brother supports her in her error, then he is doing wrong. If cohabitation becomes impossible and reconciliation efforts fail, divorce might be the best option. If the harm is not from the husband, then he has the right to refuse to divorce her until she ransoms herself from him with money. The custody of the daughter goes to her mother after divorce, unless there is an impediment. A sudden change in disposition may indicate magic (sihr), possession (mass), or evil eye (ayn), and one should be diligent in performing ruqyah (definition) (Islamic exorcism).