about imap-login: Error and imap: Error after Version-UP

Mon Nov 16 13:44:22 EET 2020

We have upgraded dovecot from 2.0.9 (centos6) to 2.2.36 (centos8).

If more than 1000 users log in, the following error will occur.
-------------------------
Nov 16 11:33:00 xxxxxxxx dovecot[1361]: imap-login: Error: master(imap):
net_connect_unix(imap) failed: Resource temporarily unavailable -
http://wiki2.dovecot.org/SocketUnavailable (client-pid=1362,
client-id=129834, rip=10.10.10.10, created 551 msecs ago, received 0/4
bytes)
Nov 16 11:30:26 xxxxxxxx dovecot[1361]: imap: Error:
net_connect_unix(/var/run/dovecot/stats-writer) failed: Resource temporarily
unavailable
-------------------------

I set the following because I thought it was a problem with the number of
connections, but I get an error.

service imap-login {
  service_count=0
  client_limit = $default_client_limit
  process_min_avail = 4
  process_limit = $default_process_limit
  vsz_limit = 1G
}

limits.conf
-------------------------
* soft nofile 4096
* hard nofile 4096
-------------------------

cat /proc/sys/net/core/somaxconn
-------------------------
4096
-------------------------

Is there any other tuning point?
Where should I look?