Postgres is really picky about queries containing GROUP BY. Every column which isn't listed in the GROUP BY clause has to be wrapped up in an aggregate function. The problem was that we were generating queries like this: SELECT karmastats.receiver AS karmastats_receiver, sum(karmastats.kcount) AS anon_1 FROM karmastats GROUP BY karmastats.receiver ORDER BY karmastats.kcount Note the unadorned karmastats.kcount in the ORDER BY clause. The fix is to use sum(karmastats.kcount) everywhere.
As noted in the README, this practice is no longer necessary. The plugin loader will instantiate each plugin.