dovecot: auth: Error: policy(?,xx.xx.xx.xx): Policy server HTTP error: Connection lost: read(127.0.0.1:579) failed: EOF (Request queued 2.784 secs ago, 1 attempts in 2.784 secs, 2.784 in other ioloops, connected 21.780 secs ago)

This error is caused by cPhulkd of cPanel, so you can temporally disable it, while investigating.

Than check for errors:
/usr/local/cpanel/logs/cphulkd_errors.log
/usr/local/cpanel/logs/cphulkd.log