Hi Vincent,
Just going to start with ruling out some of the initial possibilities. First, what version of MySQL do you have installed? Both of these queries use recursive methods that are only supported in MySQL 8.0, which is what we recommend installing in the documentation. If you have tried upgrading in place and reusing an older MySQL version (such as 5.7), then you might see these types of errors.
Did you load your 2.5.4 data into your new 2.6 installation? If yes, did you remember to follow all the steps in our Upgrading documentation, including dropping and re-creating the database, running the upgrade tasks, and restarting services? See:
Cheers,