I've noticed issues with the WPMU Dashboard plugin when installed on a couple of dev sites (for same client... just debugging issues).
When I submit content I get blank pages. When I try to submit new product categories for MarketPress I get errors listing an array of some sort. There are also misc. blank pages when trying to update system settings, etc.
My development on this site was at a stand still until I disabled the WPMU Dev Dashboard plugin.
Note: I installed on a clean install to verify and I've confirmed it is the WPMU Dev Dashboard plugin.