@saphiloche I think I found the issue here so might be worth you checking it out or your developer / web guy if you have one?
It seems the problem is with the Woopress update 3.0, it seems to load more header information compared to 2.9 version, I had header info in my child theme so I removed this, so it seems that anything header related will conflict with the new theme update.
Might be worth looking at any additional functions you might have added, for now I have Woopress 3.0 working with Woocommerce 3.0.4
I do have some issues with the theme update that I need 8 theme to address though.
Also 8 theme documentation on these major updates could be better IMO