I’m posting on this topic in case anyone replies with a possible solution. The same thing is happening on a site for me when I have it set to update the cart without leaving the product page. No errors show and I’ve tried the usual of turning off all other plugins and switching to a default theme.
On another site almost same issue. It does update the cart but the cart widget doesn’t update the items unless you refresh the page. The only error is shows is a javascript a is undefined. Tried the same there – turn off all other plugins and switched to default theme.
The second issue can be seen here: http://www.eventsunlimited.com/game-rentals/game-rooms-arcade/arcade-games/big-buck-hunter/
But the first one, I had to change the setting to go right to the cart when adding an item so customers could use the site. So you can’t see the issue there right now.
