Any Known Incompatibilities with CiviVolunteer and Other Extensions?
The answer and patch at Error with CiviVolunteer after upgrading to PHP 7.1 did have the answer after all. Thanks, Jon, for causing me to take a second look at that!

On our WordPress installation, the file is located here:

/public_html/wp-content/uploads/civicrm/ext/org.civicrm.volunteer/CRM/Volunteer/Permission.php


The patch required changing two lines: the class declaration at the top of the file and the return value of the check function.

I still don't know what it was reporting an error on line zero, but all's well. And I hope that CiviVolunteer gets an update soon that applies the patch! Kudos to Tim Otten for creating the patch.