i cant add products to basket anymore

I havent edited any of the zen cart files lately except trying to fix issues with paypal and added few lines to chemo's seo contribution but i tried with the original files as well and it didnt help anything. when i try to add a product it just displays the frontpage and it doesnt really do anything the url shows this

http://www.cccc.com/shop/product_info?products_id/306/shop/in-my-head-book-p-306.html/action=add_product

and i'm completely lost... I've removed everything i've added recently and tried with earlier files and now i've run out of the ideas. If I disable the seo from admin every product says product not found when try to add it to basket

i disabled Use Search-Engine Friendly URLs from mystore and it did the trick. So now i'm wondering does it make difference for search engines is the url like
nameproductype fex. carpartstiresmichelinwinter or like car-parts-tires-michelin-winter since I usually use product name and manufacturer etc in the product name and it looks bit stupid if their all typed in a row. all the spaces used to be separated with - but apparently using different seo mods occured to be bit problematic