What db permissions are needed for pgAgent user?
Had this same issue!

Looking in the postgres log it appeared pgagent was failing to invoke one of the sequences in order to add entries into the job log.  Once I did the below (in postgres 9.3) and restarted my pgagents, all worked well:

GRANT ALL ON ALL SEQUENCES IN SCHEMA pgagent TO pgagent_user;
GRANT ALL ON ALL FUNCTIONS IN SCHEMA pgagent TO pgagent_user;