Categories not saving

Hi,
I’m trying to use an plugin to create aHover in the images of my site. I installed ‘Rd hover effects’ on ‘make’ theme (standard) and everything seems to be fine. The problem comes when I try to create a category. It looks like it was created but when I reload the categories page or go the page I want to insert the roll over, it has just disappeared (no warning or messages at all). Actually, I can see how under the categories list it shows ’10 items’ but it doesn’t list them. In some way they’re there but I can’t see them.

My knowledge on WP is quite limited but reading some posts here I found this (https://www.ads-software.com/support/topic/unable-to-create-categories-1). I changed the settings/general so my site starts with ‘www.’ and back but still nothing.

Did anyone have to solve something like that? I would appreciate any advise or recommendation. Thanks very much in advance!