I built a fairly site WordPress around two years ago, then some time later added a few products for sale using the WP-Ecommerce plugin, and all was well.
Recently I found that when someone visited the site for the first time (in that browser on that device), the first page loaded fine, but any subsequent pages would simply display an error message.
Firstly I deactivated all the plugins through the database and the site worked again. Then I reactivated the plugins one by one until it broke again, and the plugin that seemed to do this was the SEO plugin, SEO Ultimate (http://www.seodesignsolutions.com/wordpress-seo/). As I've recently started to use the alternative plugin, WordPress SEO by Yoast (http://yoast.com/wordpress/seo/) instead, I simply uninstalled SEO Ultimate and installed WordPress SEO by Yoast instead, and all seemed well again...
... until today! The issue whereby only the first page requested would display returned. Again, I disabled all the plugins, but this time I reactivated WP-Ecommerce last, and that seemed to cause the problem. They I noticed a new update for the plugin which promised to "Fix: Fatal error in wpsc_product_list_exclude_child_categories() due to other themes or plugins calling get_posts() before the main query is set up", which sounded a bit like my problem, and would explain why different plugins seemed able to cause the problem, so I updated the plugin, confident this was it, only to find that reactivating the plugin caused the site to stop working after the first page again, sometimes, whilst other times it displays this message and leaves it deactivated:
Plugin could not be activated because it triggered a fatal error.
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 1966080 bytes) in /var/www/vhosts/6880.co.uk/httpdocs/wp-content/plugins/wp-e-commerce/wpsc-updates/currency_list.php on line 187
For now, I have left the WP-Ecommerce plugin deactivated, and hidden all the ecommerce pages from the menu, as luckily the ecommerce side of this site is not really very important.
I'm happy to provide any further information you may need, to give temporary access to the site, or to do anything else that may be needed to assist you with resolving this issue.