TL;DR: Their API is down and there is nothing I can do about it.

I just want to put this here as I know some of you may experience the NHentai option not working.
Currently, the NHentai API is in a state of randomly working and then suddenly stopping. I'm not sure why it's happening, but it is. There is a post on r/NHentai about this which you may want to read. It's not all bad news as sometimes it does work--you just have to be there at the right time.
I wish I could do something about it but sadly it's out of my control. We just have to wait for it to be fixed... talk about bad timing!