403 error when viewing posts through categories

[hypman 9]

Hi My blog posts show up on my blog page but when i click categories, and try and view the same blog posts i get a 403 error? http://stopsmokingderbys.co.uk/hypnosis-derby-blog/ Ive got a feeling its to do with the yoast seo plugin, as the URL i just tried came back with .html on the end? How can i change this to work in both circumstances? thanks

[hypman 9]

Ive done that, by altering the permalink settings.