I faced same problem, the reason I could diagnose was that I have deactivated MarketPress Lite plug-in but a Product Category created in MarketPress Lite was used in Menu. (Custom Menus also failed to open.)
I simply reactivated the Plug-in, opened custom menus, deleted the menu item which was based on MarketPress Lite Product Category, deactivated MarketPress Lite.
Every things work fine now
ASPatti