I could to reply to the original thread:
https://www.mail-archive.com/dovecot@dovecot.org/msg85204.html

But I managed to get a stack trace of the error:

root@mail:~ # lldb /usr/local/libexec/dovecot/auth
(lldb) target create "/usr/local/libexec/dovecot/auth"
Current executable set to '/usr/local/libexec/dovecot/auth' (x86_64).
(lldb) run
Process 54354 launched: '/usr/local/libexec/dovecot/auth' (x86_64)
Error: stats: open(old-stats-user) failed: No such file or directory
Panic: file http-client.c: line 646 (http_client_context_close): assertion failed: (cctx->clients_list == NULL)
Process 54354 stopped
* thread #1, name = 'auth', stop reason = signal SIGABRT
    frame #0: 0x000000080159f33a libc.so.7`__sys_thr_kill + 10
libc.so.7`__sys_thr_kill:
->  0x80159f33a <+10>: jb     0x80159d498
    0x80159f340 <+16>: retq  
    0x80159f341:       int3  
    0x80159f342:       int3  
(lldb) bt all
* thread #1, name = 'auth', stop reason = signal SIGABRT
  * frame #0: 0x000000080159f33a libc.so.7`__sys_thr_kill + 10
    frame #1: 0x0000000801517c74 libc.so.7`__raise + 52
    frame #2: 0x00000008015c9109 libc.so.7`abort + 73
    frame #3: 0x00000008012e606b libdovecot.so.0`default_fatal_finish(type=LOG_TYPE_PANIC, status=0) at failures.c:465:3
    frame #4: 0x00000008012e3f7c libdovecot.so.0`fatal_handler_real(ctx=0x00007fffffffe610, format="file %s: line %d (%s): assertion failed: (%s)", args=0x00007fffffffe5f0) at failures.c:477:2
    frame #5: 0x00000008012e3f14 libdovecot.so.0`default_fatal_handler(ctx=0x00007fffffffe610, format="file %s: line %d (%s): assertion failed: (%s)", args=0x00007fffffffe5f0) at failures.c:485:2
    frame #6: 0x00000008012e4253 libdovecot.so.0`i_panic(format="file %s: line %d (%s): assertion failed: (%s)") at failures.c:530:2
    frame #7: 0x000000080124f2ed libdovecot.so.0`http_client_context_close(cctx=0x000000080205c048) at http-client.c:646:2
    frame #8: 0x000000080124ef22 libdovecot.so.0`http_client_global_context_ioloop_switched(prev_ioloop=0x0000000802030640) at http-client.c:710:3
    frame #9: 0x00000008013102e4 libdovecot.so.0`io_loop_set_current(ioloop=0x0000000000000000) at ioloop.c:982:4
    frame #10: 0x00000000010aa0be auth`driver_pgsql_set_state(db=0x0000000802092200, state=SQL_DB_STATE_IDLE) at driver-pgsql.c:103:3
    frame #11: 0x00000000010a9ff0 auth`connect_callback(db=0x0000000802092200) at driver-pgsql.c:206:3
    frame #12: 0x000000080130f3b2 libdovecot.so.0`io_loop_call_io(io=0x00000008020105c0) at ioloop.c:737:2
    frame #13: 0x0000000801313e0b libdovecot.so.0`io_loop_handler_run_internal(ioloop=0x0000000802030640) at ioloop-kqueue.c:164:4
    frame #14: 0x000000080130fa15 libdovecot.so.0`io_loop_handler_run(ioloop=0x0000000802030640) at ioloop.c:789:2
    frame #15: 0x000000080130f8fa libdovecot.so.0`io_loop_run(ioloop=0x0000000802030640) at ioloop.c:762:3
    frame #16: 0x00000000010a7eef auth`driver_pgsql_wait(_db=0x0000000802092200) at driver-pgsql.c:1279:3
    frame #17: 0x00000000010a4c59 auth`sql_wait(db=0x0000000802092200) at sql-api.c:820:3
    frame #18: 0x00000000010a7fc0 auth`driver_pgsql_get_flags(db=0x0000000802092200) at driver-pgsql.c:306:3
    frame #19: 0x00000000010a2fa9 auth`sql_get_flags(db=0x0000000802092200) at sql-api.c:171:10
    frame #20: 0x00000000010a585e auth`driver_sqlpool_get_flags(_db=0x0000000802092000) at driver-sqlpool.c:450:9
    frame #21: 0x00000000010a2fa9 auth`sql_get_flags(db=0x0000000802092000) at sql-api.c:171:10
    frame #22: 0x000000000108c675 auth`passdb_sql_init(_module=0x0000000802041ca8) at passdb-sql.c:281:10
    frame #23: 0x000000000107d577 auth`passdb_init(passdb=0x0000000802041ca8) at passdb.c:247:3
    frame #24: 0x0000000001052a89 auth`auth_passdb_init(passdb=0x0000000802041ba8) at auth.c:308:2
    frame #25: 0x0000000001051dbc auth`auth_init(auth=0x0000000802041b70) at auth.c:322:3
    frame #26: 0x0000000001051d21 auth`auths_init at auth.c:427:3
    frame #27: 0x0000000001050610 auth`main_init at main.c:242:2
    frame #28: 0x00000000010503ad auth`main(argc=1, argv=0x00007fffffffeaf8) at main.c:391:2
    frame #29: 0x000000000104ff7d auth`_start(ap=<unavailable>, cleanup=<unavailable>) at crt1_c.c:75:7