Thank you for reporting this issue, which fts driver are you using? Can you provide `doveconf -n` output?

Seems like I'm using ... squat, and this, in turn, seems to be the old legacy piece from who knows how long ago installed instance. From your question - am I right supposing that if I will get rid of squat things will improve ? Also seems like it's worth mentioning that downgrading to dovecot 2.3.15 improved things radically - no crash at this time (with the same set of configuration files).

dovecot -n output:

===Cut===

# doveconf -n
# 2.3.15 (0503334ab1): /usr/local/etc/dovecot/dovecot.conf
# Pigeonhole version 0.5.15 (e6a84e31)
# OS: FreeBSD 13.1-RELEASE amd64   
# Hostname: it-r-support
auth_debug = yes
auth_debug_passwords = yes
auth_mechanisms = plain login cram-md5 digest-md5
auth_verbose = yes
auth_verbose_passwords = yes
default_client_limit = 40963
default_process_limit = 10240
disable_plaintext_auth = no
first_valid_gid = 1143
first_valid_uid = 1143
hostname = alamics.ru
mail_home = /var/imap/%d/%n
mail_location = maildir:~/Maildir
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 index ihave duplicate mime for
everypart extracttext
mbox_write_locks = fcntl
namespace {
 inbox = yes
 list = yes
 location =  
 prefix =  
 separator = /
 subscriptions = yes
 type = private
}
namespace {
 location = maildir:/var/imap/%d/public
 prefix = Public/
 separator = /
 subscriptions = no
 type = public
}
namespace {
 list = children
 location = maildir:%%h/Maildir:INDEX=~/Maildir/shared/%%u
 prefix = shared/%%u/
 separator = /
 subscriptions = no
 type = shared
}
passdb {
 args = /usr/local/etc/dovecot/dovecot-sql.conf.ext
 driver = sql
}
plugin {
 acl = vfile
 acl_shared_dict = file:/var/db/dovecot/shared-mailboxes
 fts = squat
 fts_autoindex = yes
 fts_squat = partial=4 full=10
 quota = maildir:User quota
 quota_rule = *:storage=50G
 quota_rule2 = Trash:storage=+10%%
 quota_rule3 = Spam:storage=+20%%
 quota_warning = storage=95%% quota-warning 95 %u
 quota_warning2 = storage=80%% quota-warning 80 %u
 sieve = ~/.dovecot.sieve
 sieve_after = /var/imap/sieve/sieve-after
 sieve_before = /var/imap/sieve/sieve-before
 sieve_default = /var/imap/sieve/default.sieve
 sieve_dir = /var/imap/%d/%n/sieve
 sieve_global_dir = /var/imap/sieve
 sieve_max_script_size = 1M
}
postmaster_address = postmaster@alamics.ru
protocols = imap pop3 lmtp sieve
service auth {
 client_limit = 62464
 unix_listener /var/spool/postfix/private/auth {
   group = postfix
   mode = 0666
   user = postfix
 }
 unix_listener auth-userdb {
   group = dovecot
   mode = 0660
   user = dovecot
 }
}
service imap-login {
 client_limit = 10000
 inet_listener imap {
   port = 143
 }
 inet_listener imaps {
   port = 993
   ssl = yes
 }
 process_limit = 10240
 process_min_avail = 1
 service_count = 0
 vsz_limit = 256 M
}
service imap {
 drop_priv_before_exec = yes
 process_limit = 10240
}
service managesieve-login {
 inet_listener sieve {
   port = 4190
 }
}
service pop3-login {
 inet_listener pop3 {
   port = 110
 }
 inet_listener pop3s {
   port = 995
   ssl = yes
 }
}
service quota-warning {
 executable = script /usr/local/bin/quota-warning.sh
 unix_listener quota-warning {
   user = dovecot
 }
 user = dovecot
}
ssl_cert = </usr/local/etc/letsencrypt/live/alamics.ru/fullchain.pem
ssl_key = # hidden, use -P to show it
syslog_facility = local6
userdb {
 args = /usr/local/etc/dovecot/dovecot-sql.conf.ext
 driver = sql
}
protocol lmtp {
 mail_plugins = " quota acl sieve"
}
protocol lda {
 mail_plugins = " quota acl sieve"
}
protocol imap {
 imap_client_workarounds = tb-extra-mailbox-sep
 mail_max_userip_connections = 512
 mail_plugins = quota imap_quota fts fts_squat
}

===Cut===

Thanks.

Eugene.