When searching genre or category or any combo, if there are more than 50 pages of results you can't go beyond that first 50 pages. You can change how many manga are seen on a page to 100, but if it's still more than 50 pages you still can't see what's beyond it. This seems to only happen with genre and category search, it doesn't happen when you're searching for a title.

Example:

Clicking just Action in advanced genre search and setting it so it shows 100 manga per page will give you 84 pages of manga and clicking Last will give you an error page saying "Please make your query more restrictive."

It's the same when you click the category: "Collection of Stories" except it has 51 pages of manga.

And then when you search "Jin" and click to see more results you get 64 pages of 100 per page and you get to see all the pages.

I ran into this issue because I wanted to see what the oldest shoujo manga on the site was but clicking year only orders from most recent to oldest and so I clicked last and wound up with an error.

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I can confirm this too. Complex searches where you "just" exclude anything (as genre and demographic) you wouln´t want to read don´t quite work with the MU search engine.
I would suggests to search with only 1 demographic, while excluding the other 3. So do the same search 4 times, just with different demos, to keep the result count below 50 pages.
Further set the Items Per Page to 100 in your profile. It will take way more possible results to go over 50 pages with that trick.

I also read EU/US comics and am a librarian.
Manga-Masters, My ANN-Lists + Imdb

I don't run into this issue often (since I'm not usually looking beyond 50 pages), only when I'm looking for random things like the oldest shoujo manga on this site which when excluding everything except shoujo still gives more than 50 pages of manga so yeah, not really an issue that can be solved without a fix. (Though I guess I could go add the first 10 pages of results into a randomly created list and then exclude everything on my lists, but that's a bit...)

Must Read Webcomic:
Unsounded

Lists:
Reading
Complete
Western Webcomics