I have found - maybe - an interim solution:
Disable “customer must be logged in to checkout”
This way it doesn’t attempts to update the table before the new
customer actually completes the form (which is, i think, part of the issue)
I would love to have a better solution though
Also noticed that setting the “Enable One Page Checkout” option to “No” removes completely the ability to check out.
Is that working as designed? Such option made me think there would be the possibility to
either use the one page checkout or a multi page checkout