In late 2016, Mozilla shut down their persona service. This results in new backpack connections being unable to authenticate against backpack.openbadges.org.
The openbadges backpack has been migrated to a new authentication service, but Moodle is still calling the deactivated persona service for login.
Follow this issue on the Tracker for more information. Please vote for MDL-57429 if it affects you.
Good news! This problem has now been fixed and the solution was integrated into MOODLE_31_STABLE and MOODLE_32_STABLE on 23rd March 2017.
Great news!
Then the information on MoodleDocs, https://docs.moodle.org/32/en/Badges_settings#Enable_connection_to_external_backpacks needs to be changed.
Does it mean I need to upgrade the Moodle to support it? or can I use the old Moodle 2.8 for that and just update the plugin?