On 11 Sep 2018, at 10.57, Simone Lazzaris <s.lazzaris@interactive.eu> wrote:

Sep 11 03:25:55 imap-front4 dovecot: director: Panic: file doveadm-connection.c: line 1097 (doveadm_connection_deinit): assertion failed: (conn->to_ring_sync_abort == NULL)
Sep 11 03:25:55 imap-front4 dovecot: director: Fatal: master: service(director): child 4395 killed with signal 6 (core dumps disabled)

It's crashing. Can you get gdb backtrace? First enable core dumps. https://dovecot.org/bugreport.html#coredumps