Solved: CiviRules 2.1 and CiviVolunteer 4.7.31-2.3.1 not working with CiviCRM 5.4.0
Many post-upgrade issues like this can be resolved by clearing the caches.  It should be taken care of by the upgrade process, but it's worth trying it again.

You can clear the caches in various ways including: Administer &gt; System Settings &gt; Cleanup Caches and Update Paths &gt; Cleanup Caches