I think i found a rare problem with the get_contexts_for_userid in bigbluebutton. This may be linked to this old entry: https://tracker.moodle.org/browse/CONTRIB-7915
If there is no user found for the given id the function returns null. But Moodle dont accept a null return value from this function. The error occurs during the cron processing of the privacy data. The message is:
"Return value of mod_bigbluebuttonbn\privacy\provider::get_contexts_for_userid() must be an instance of core_privacy\local\request\contextlist, null returned"
I cant not give reproduction infos. I am not sure how to trigger this call with a userid which may not be found on purpose. You would have to make an privacy call for an already fully deleted user.
I suppose this function should return an empty context list instead of a null.