At some point, the Open Data Commons website updated their site structure and the ArcGIS Hub link no longer works.
Has PDDL link to this page: https://opendatacommons.org/licenses/pddl/summary
<Error><Code>NoSuchKey</Code><Message>The specified key does not exist.</Message><Details>No such object: opendatacommons.org/licenses/pddl/summary</Details></Error>
But it should be linking to https://opendatacommons.org/licenses/pddl/summary/index.html
That link is automatically created just by typing in PDDL. Hopefully the right person will see this and update the backend to point to the correct page.