I think that yoast plugin has a problem or conflict with wordpress e-commerce plugin. I've an e-commerce with 3500 products. When I try to access to section products in the backend, wordpress has a fatal error: Fatal error: Out of memory (allocated 95944704) (tried to allocate 35 bytes) in /homepages/.../cache.php on line 572.
This is not a memory problem. When all is active, the sistem need around 64MB and I've 128MB. When I disable the Yoast plugin and enable other SEO plugin all is ok.
Sorry for my english ;-) Thanks in advance.