The Requirements link on the 3.0 To 3.1 Upgrade page gives a 404 error. The page with the link is at:

https://www.phpbb.com/support/docs/en/3.1/ug/upgradeguide/upgrade3/

The referenced link that fails is:

https://www.phpbb.com/support/docs/en/3.1/ug/upgradeguide/quickstart/quick_requirements.php

Being able to check requirements is obviously necessary, so this is a Critical error.