WordPress.org

Every page on my site respond with 404 error

Go to Options/Permalinks page in Site Admin and click Save Changes to re-generate permalinks.

First category is used for permalinks instead of selected

Check the permalinks structure you used (Options/Permalinks). You should use %scategory% instead of %category%.

I love this plugin. How to do a favor to the author?

Just vote for this plugin on WordPress.org. And thank you!