It's hard to tell what I'm looking at (maybe it's the beer??) but seems like there's a big hole. Do you have better pictures?
The usual failure mode for the expansion tank is a hairline crack--one that you won't usually see with the naked eye. But at the rate you're spilling, seems something came loose. any recent work on the system?
Also there's no bleed valve at the bottom of the et.
Tip for next time taking the expansion tank out, get a big heavy duty screwdriver and pry from the bottom of the tank (against the bracket) and you can get the tank out in less than 5 seconds using leverage