Has bathing or showering at sunset been forbidden?

There is no authentic (sahih) or weak (da'if) hadith that prohibits bathing at sunset, and none of the scholars have ever stated such a thing. Some jurists disliked entering public baths near sunset because it is a time when devils spread, and due to the potential for uncovering 'awrat (private parts) therein, whereas the Hanbalis did not dislike it.