By removing the word “category” in the URL of my site I think I discovered a bug in version 3.9.1.

“Settings -> Permalinks” the “category base” using a point.

From ejemplo.org / category / categoryname

A ejemplo.org / categoryname

It worked perfect.

But click a category (in the menu) with a word ending in “s” … a 404 error appears.

By changing the word “audios” with “audiosa” does not end in “s” (in the slug of the category)

It worked perfect.

Possible cause and solution?

Thank you.

ps: I apologize for my English.