fts_solr: Error: fts_solr: received invalid uid '0'

[Fabian]

Hi,

we are trying to add full text search functionality with Solr to our Doveoct setup. Our Versions:
OS: Debian 9
Tried versions: 
- Dovecot 2.2.7 with Solr 3.6
- Dovecot 2.3.4 with Solr 8.2
(2.2.7 from offical Debian repository, 2.3.4 from backports)

Search is working mostly of the time perfrectly smooth. But sometimes following message appears in mail.err:
dovecot: imap(username)<16189><UxYWLVuSYMasEQoK>: Error: fts_solr: received invalid uid '0'

If this error occurs our webmail frontend delivers most of the time a timeout. Sometimes the search only takes really long.

Are  there any ideas why this error occurs? We are not able to reproduce the error in such a way that it would always be reproducible. However, we can reproduce the behavior in some form over and over again - but we do not know exactly what is decisive.

If you need any further information please ask - thanks! :-)

dovecont -n:
2.3.4.1 (f79e8e7e4): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.5.4 ()
# OS: Linux 4.9.0-11-amd64 x86_64 Debian 9.11 ext4
# Hostname: localhost
auth_master_user_separator = *
auth_username_format = %Ln
default_client_limit = 5000
default_process_limit = 1000
default_vsz_limit = 256M
dict {
  sqlquota = mysql:/etc/dovecot/dovecot-dict-sql.conf.ext
}
disable_plaintext_auth = no
first_valid_uid = 480
imap_capability = +XDOVECOT
lmtp_save_to_detail_mailbox = yes
mail_attribute_dict = file:/var/spool/dovecot/user/%n/Maildir/dovecot-attributes
mail_gid = vmail
mail_location = maildir:/var/spool/dovecot/user/%d/%n/Maildir:LAYOUT=fs
mail_plugins = " listescape mail_log notify quota acl fts fts_solr virtual"
mail_privileged_group = vmail
mail_uid = vmail
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 foreverypart extracttext
namespace {
  hidden = yes
  inbox = yes
  list = no
  location = 
  prefix = 
  separator = /
  subscriptions = yes
}
namespace {
  list = yes
  location = maildir:%%h/Maildir:INDEXPVT=~/shared/%%u:LAYOUT=fs
  prefix = shared/%%n/
  separator = /
  subscriptions = no
  type = shared
}
namespace Public {
  list = children
  location = maildir:/var/spool/dovecot/public:INDEXPVT=~/public:LAYOUT=fs
  prefix = Public/
  separator = /
  subscriptions = no
  type = public
}
namespace Virtual {
  hidden = no
  list = yes
  location = virtual:/etc/dovecot/virtual:INDEX=/var/spool/dovecot/user/%u/virtual
  prefix = Virtual/
  separator = /
  subscriptions = no
}
namespace inbox {
  inbox = no
  location = 
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix = INBOX/
  separator = /
  subscriptions = no
  type = private
}
passdb {
  args = /etc/dovecot/masteruser.%s
  driver = passwd-file
  master = yes
}
passdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
passdb {
  args = scheme=CRYPT username_format=%u /etc/dovecot/users
  driver = passwd-file
}
plugin {
  acl = vfile
  acl_anyone = allow
  acl_shared_dict = file:/var/spool/dovecot/public/shared-mailboxes.db
  fts = solr
  fts_autoindex = yes
  fts_solr = url=<a href="http://172.17.10.12:8983/solr/dovecot/" rel="noreferrer noopener" target="_blank">172.17.10.12:8983/solr/dovecot/</a>
  quota = dict:User quota::no-unset:proxy::sqlquota
  quota_rule = *:storage=10G
  sieve = file:~/sieve;active=~/.dovecot.sieve
  sieve_before = /var/spool/dovecot/globalsieve/duplicate.sieve
  sieve_duplicate_default_period = 1h
  sieve_duplicate_max_period = 1d
}
protocols = " imap lmtp sieve pop3"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0660
    user = postfix
  }
}
service decode2text {
  executable = script /usr/lib/dovecot/<a href="http://decode2text.sh" rel="noreferrer noopener" target="_blank">decode2text.sh</a>
  unix_listener decode2text {
    mode = 0666
  }
  user = dovecot
}
service dict {
  unix_listener dict {
    group = vmail
    user = vmail
  }
}
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}
service managesieve-login {
  inet_listener sieve {
    port = 4190
  }
}
ssl = no
userdb {
  args = /etc/dovecot/mysql-userdb.conf.ext
  driver = sql
}
protocol lmtp {
  mail_plugins = " listescape mail_log notify quota acl fts fts_solr virtual acl quota sieve notify"
  quota_full_tempfail = yes
}
protocol imap {
  imap_capability = +XDOVECOT
  mail_max_userip_connections = 100
  mail_plugins = " listescape mail_log notify quota acl fts fts_solr virtual imap_acl imap_quota"
}
remote <a href="http://172.17.10.10" rel="noreferrer noopener" target="_blank">172.17.10.10</a> {
  protocol imap {
    imap_metadata = yes
  }
}