[Plugin: WordPress SEO by Yoast] Redirects not working

I installed WordPress SEO 1.1.9
I asked it to change permalinks.
Some links are not redirecting.
On an outside website this link http://englishforuniversity.com/?cat=109
is not found. It looks for this page and says it is not found: http://englishforuniversity.com/category/paraphrasing/

But within my own website the link works and comes up with:
http://englishforuniversity.com/paraphrasing/

But ticking the box ‘strip the category base from url’ doesn’t make it work either – it makes the page totally unreachable from everywhere

http://ww.wp.xz.cn/extend/plugins/wordpress-seo/