500 - Internal server error - only with Categories with Market Press


It seems that every time I update the mareketpress plug-in it starts throwing error 500's with 'category' pages, or add to cart button.

I have the same issue with two sites. http://www.bottlewithwaterfilter.com and http://www.coastguardflags.com

Each throws the 500 error for different reasons. I have gone so far as to: a) diasable ALL plug-in's except MarketPress, still the same issue. b) incrementally disable plug-ins (no effect) c) removed or deactivated 'advanced-cache.php' incase that was an issue (showing as a drop-in plug-in). d) renamed the customized url structure for the store (no effect) and many more adjustments. Nothing works.

the products show on some pages, but any further action, like 'add to cart' or navigation to a 'category' give the 500 error.

I am running all sites on IIS 8 on Server 2012 Standard. I have full server access. I have both sites running in dedicated application pools. There is plenty of memory and every other site runs with-out issue. I host several ASP based shopping carts, that require IIS and SQL. I have 10 other WordPress sites all running. Market Press seems to be giving me issues.

It ran great for a month, then when it was updated the issues started. I like the module, and the overall concept of having a store in WordPress is appealing, but I am losing faith in the stability.

I am at a loss.

Please help.