How is it going? Are the developers getting anything figured out on this? (see message from Adam below):
I just had a long conversation with one of the developers, referring all the issues and problems identified to him. The reason an update is breaking the site is unfortunately the GridMarket theme. That's the theme that was designed a long time ago for MarketPress previous releases (before MarketPress 3.xxxx came out).
That's a legacy theme and as such is no longer receiving any maintenance updates, only security fixes. We've moved focus to Upfront but kept these legacy themes as there are some members still using them regardless of updates.
Given the situation here and that I dropped the ball on figuring this out earlier, I asked one of our developers to take a look at this and he's now working on fixing this up for you.
I've also sent you an important e-mail regarding this situation and your account. Please take a look into your e-mail inbox.