Since I upgraded to 2.3.4 I have lots of line in logs like these:
 
Nov 28 17:35:48 buzalum_rhel7 dovecot: imap(iniramgon@mydomain)<udqWKLx7HN/AqAFh>: Error: unlink(/etc/dovecot/virtual/dovecot-acl-list) failed: Read-only file system (in acl-backend-vfile-acllist.c:338)
Nov 28 17:35:48 buzalum_rhel7 dovecot: imap(iniramgon@mydomain)<udqWKLx7HN/AqAFh>: Error: dovecot-acl-list creation failed: safe_mkstemp(/etc/dovecot/virtual/.temp.buzalum_rhel7.6469.) failed: Read-only file system
 
These lines did not happen with Dovecot 2.3.2.1. doveconf -n output below. I have also tested to create an emty file with userid entrega in /etc/dovecot/virtual/ and works, so it does not seem a problem with permissions. Any idea about this problem?
 

# OS: Linux 3.10.0-862.14.4.el7.x86_64 x86_64 Red Hat Enterprise Linux Server release 7.5 (Maipo) xfs
# Hostname: buzalum_rhel7
auth_cache_negative_ttl = 10 secs
auth_cache_size = 10 M
auth_cache_ttl = 2 mins
auth_master_user_separator = *
auth_mechanisms = plain login
auth_worker_max_count = 90
base_dir = /var/run/dovecot/
default_client_limit = 8192
default_process_limit = 4500
disable_plaintext_auth = no
imap_capability = +XLIST
imap_client_workarounds = tb-extra-mailbox-sep delay-newmail tb-lsub-flags
imap_id_log = *
lda_mailbox_autocreate = yes
lda_mailbox_autosubscribe = yes
mail_fsync = never
mail_gid = entrega
mail_home = /buzones/mydomain/%2.26Hn/%2.200Hn/%n/home_usuario/
mail_location = mdbox:/buzones/mydomain/%2.26Hn/%2.200Hn/%n:INDEX=/indices_dovecot/indices/%2.26Hn/%2.200Hn/%n:ALT=/buzones/alternativo/mydomain/%2.26Hn/%2.200Hn/%n
mail_log_prefix = "%s(%u)<%{session}>: "
mail_max_userip_connections = 15000
mail_plugins = " virtual zlib quota acl mail_log notify"
mail_uid = entrega
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character vacation subaddress comparator-i;ascii-numeric relational regex imap4flags copy include variables body enotify environment mailbox date
mdbox_rotate_interval = 1 days
mdbox_rotate_size = 60 M
namespace {
  inbox = yes
  location =
  mailbox Drafts {
    auto = subscribe
    special_use = \Drafts
  }
  mailbox SPAM {
    auto = subscribe
    special_use = \Junk
  }
  mailbox Sent {
    auto = subscribe
    special_use = \Sent
  }
  mailbox Trash {
    auto = subscribe
    special_use = \Trash
  }
  prefix =
  type = private
}
namespace {
  list = yes
  location = mdbox:/buzones/mydomain/%%2.26Hn/%%2.200Hu/%%n:ALT=/buzones/alternativo/mydomain/%%2.26Hn/%%2.200Hn/%%n:INDEX=/indices_dovecot/indices/%%2.26Hn/%%2.200Hn/%%n:INDEXPVT=~/mdbox/carpeta_compartida/%%u
  prefix = carpeta_compartida/%%u/
  separator = /
  subscriptions = no
  type = shared
}
namespace {
  list = yes
  location = virtual:/etc/dovecot/virtual:INDEX=~/mdbox/virtual:CONTROL=~/mdbox/virtual
  prefix = Carpetas_virtuales/
  separator = /
}
passdb {
  args = /etc/dovecot/deny
  deny = yes
  driver = passwd-file
}
passdb {
  args = /etc/dovecot/dovecot-ldap.conf
  driver = ldap
}
plugin {
  acl = vfile
  acl_shared_dict = file:/buzones/mydomain/acls_compartidas.db
  lda_mailbox_autosubscribe = yes
  mail_log_events = delete undelete expunge copy mailbox_delete mailbox_rename
  mail_log_fields = uid box msgid size subject from
  quota = dict:Cuota de usuario::file:/buzones/cuotas/%n
  quota_rule2 = Trash:storage=+10%%
  quota_warning = storage=90%% aviso_cuota 90 %u
  sieve = /buzones/mydomain/%2.26Hn/%2.200Hn/%n/home_usuario/dovecot.sieve
  sieve_dir = /buzones/mydomain/%2.26Hn/%2.200Hn/%n/home_usuario/sieve/
  zlib_save = gz
  zlib_save_level = 9
}
pop3_no_flag_updates = yes
protocols = pop3 imap sieve
service anvil {
  client_limit = 8504
}
service auth {
  client_limit = 12000
  unix_listener auth-master {
    user = entrega
  }
  unix_listener auth-userdb {
    user = entrega
  }
  user = root
}
service aviso_cuota {
  executable = script /usr/local/bin/quota-warning.sh
  unix_listener aviso_cuota {
    mode = 0666
  }
  user = entrega
}
service config {
  process_min_avail = 1
}
service imap-login {
  executable = /usr/libexec/dovecot/imap-login
  group = dovenull
  process_limit = 8000
  process_min_avail = 1
  vsz_limit = 1 G
}
service imap {
  executable = /usr/libexec/dovecot/imap
  process_limit = 8000
}
service managesieve-login {
  executable = /usr/libexec/dovecot/managesieve-login
  inet_listener sieve {
    port = 2000
  }
  process_limit = 8000
  vsz_limit = 1 G
}
service managesieve {
  executable = /usr/libexec/dovecot/managesieve
  process_limit = 8000
}
service pop3-login {
  executable = /usr/libexec/dovecot/pop3-login
  process_limit = 8000
  process_min_avail = 1
  vsz_limit = 1 G
}
service pop3 {
  executable = /usr/libexec/dovecot/pop3
  process_limit = 8000
}
service stats {
  fifo_listener stats-mail {
    mode = 0600
    user = entrega
  }
}
ssl_ca = XXX
ssl_cert = XXX
ssl_dh = # hidden, use -P to show it
ssl_key = # hidden, use -P to show it
userdb {
  driver = passwd
}
userdb {
  args = /etc/dovecot/dovecot-ldap.conf
  driver = ldap
}
userdb {
  args = /etc/dovecot/dovecot-ldap-userdb.conf
  driver = ldap
}
verbose_proctitle = yes
protocol sieve {
  managesieve_implementation_string = dovecot
  managesieve_logout_format = bytes=%i/%o
  managesieve_max_line_length = 65536
}
protocol lda {
  hostname = mydomain
  info_log_path =
  log_path =
  mail_fsync = optimized
  mail_plugins = virtual sieve zlib quota
  postmaster_address = evcorreo@us.es
  syslog_facility = mail
}
protocol imap {
  mail_plugins = virtual zlib  quota imap_quota acl imap_acl notify mail_log
}
protocol pop3 {
  mail_plugins = zlib quota virtual
  pop3_enable_last = yes
  pop3_uidl_format = %g
}