CiviVolunteer error after upgrade to Civi 5.11.0 on joomla
The warning you describe is fixed in the version of CiviVolunteer on github but they haven't tagged a release. The error sounds slightly different &amp; turning on debugging might get more info.

However I logged an issue asking them to tag a release since there are unreleased civivolunteer fixes https://github.com/civicrm/org.civicrm.volunteer/issues/508

You can get the latest civivolunteer code from github in the meantime - https://github.com/civicrm/org.civicrm.volunteer/archive/master.zip and see if that helps