BligBlueButton blocks course view

After update from Moodle 3.10.4 to Moodle 3.11 I can't view course sections below a BigBlueButton activity. If a forum and a BigBlueButton activity is in the course section 0 the nothing is shown from the course without the course section title.

The debugging shows the following message when activated:

Fatal error: Class mod_bigbluebuttonbn\completion\custom_completion contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (core_completion\activity_custom_completion::get_sort_order) in /var/www/vhosts/elisl.org/httpdocs/html/mod/bigbluebuttonbn/classes/completion/custom_completion.php on line 84

The problem happens when I open a course with a current BigBlueButton activity but also happens when I add a new BigBlueButton activity. Adding the activity works fine but saving it and going back to the course blocks everything in the current and all following sections.

I tried to deactivate the module BigBlueButton in the plugin overview. No result. All courses with a BigBlueButton activity have the same problems.
When I deactivate all instances of the module BigBlueButton in the "site administration > plugin overview" then I was able to view the complete content in all courses.

Best regards, Ralf