Hi...I resolved this problem with plugin clearing the option ''Enforce a trailing slash on all URL categories and tags'' on SEO plugin menu ''Permanent links''. Did Work to me. It's only clearing this option.
I cleared the option above and too the option enable the sitemap in the plugin. As i use another plugin for sitemap (Google XML Sitemaps Plugin), this option (sitemap) in this plugin is not necessary to me. So, I cleared both options.
This erro (erro 404 in the categories ) appeared after atualization of plugin for the last version (1.2.5).