Just upgraded to 7.x-3.4 and have everything stabalized, updated, caches cleared etc. The add to cart, checkout, and empty cart features are all working but when I try to add a product that is already in the cart OR change the quantity and then click 'update cart', I receive an HTTP 500 Error, Internal Server Error. Same thing happens when I click the 'remove' button. I've disabled all other modules on the /cart page and don't know where to go from here. Any suggestions in how I might troubleshoot it are also welcome.