Per this link,

arpad9 — Tue, 06 Mar 2012 02:27:00 +0000

Per this link, https://support.process-one.net/browse/EJAB-1463 , the plugs (flat_odbc) also require an _odbc after the name when using mySQL.

  {mod_pubsub_odbc, [
                {access_createnode, pubsub_createnode},
                {plugins, ["flat_odbc", "hometree_odbc", "pep_odbc"]}
               ]},

Works.

ejabberd - Comments for "ejabberd 2.1.5 pubsub create node crash [solved]"

Per this link,