Plugin 'sieve_imapsieve' not found

[Christopher Satchell]

Hello,

when Dovecot launches (and pretty much at any action afterwards) it 
always throws following error:

| managesieve: Fatal: Plugin 'sieve_imapsieve' not found from directory 
/usr/lib/dovecot/modules/sieve
| config: Error: managesieve-login: dump-capability process returned 89

dovecot-sieve and dovecot-managesieved are both installed and the only 
plugin in the mentioned folder is `lib90_sieve_extprograms_plugin.so`.

`doveconf -n` output:

# 2.2.22 (fe789d2): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.13 (7b14904)
managesieve(username): Fatal: Plugin 'sieve_imapsieve' not found from 
directory /usr/lib/dovecot/modules/sieve
doveconf: Error: managesieve-login: dump-capability process returned 89
# OS: Linux 4.4.0-101-generic x86_64 Ubuntu 16.04.3 LTS
auth_mechanisms = plain login
mail_gid = vmail
mail_home = /var/vmail/mailboxes/%d/%n
mail_location = maildir:~/mail:LAYOUT=fs
mail_privileged_group = vmail
mail_uid = vmail
namespace inbox {
   inbox = yes
   location =
   mailbox Drafts {
     auto = subscribe
     special_use = \Drafts
   }
   mailbox Sent {
     auto = subscribe
     special_use = \Sent
   }
   mailbox Spam {
     auto = subscribe
     special_use = \Junk
   }
   mailbox Trash {
     auto = subscribe
     special_use = \Trash
   }
   prefix =
}
passdb {
   args = /etc/dovecot/dovecot-sql.conf
   driver = sql
}
plugin {
   imapsieve_mailbox1_before = file:/var/vmail/sieve/global/learn-spam.sieve
   imapsieve_mailbox1_causes = COPY
   imapsieve_mailbox1_name = Spam
   imapsieve_mailbox2_before = file:/var/vmail/sieve/global/learn-ham.sieve
   imapsieve_mailbox2_causes = COPY
   imapsieve_mailbox2_from = Spam
   imapsieve_mailbox2_name = *
   quota = maildir:User quota
   quota_exceeded_message = Benutzer %u hat das Speichervolumen 
überschritten. / User %u has exhausted allowed storage space.
   sieve = 
file:/var/vmail/sieve/%d/%n/scripts;active=/var/vmail/sieve/%d/%n/active-script.sieve
   sieve_before = /var/vmail/sieve/global/spam-global.sieve
   sieve_global_extensions = +vnd.dovecot.pipe
   sieve_pipe_bin_dir = /usr/bin
   sieve_plugins = sieve_imapsieve sieve_extprograms
}
protocols = imap lmtp sieve
service auth {
   unix_listener /var/spool/postfix/private/auth {
     group = postfix
     mode = 0660
     user = postfix
   }
   unix_listener auth-userdb {
     group = vmail
     mode = 0660
     user = vmail
   }
}
service imap-login {
   inet_listener imap {
     port = 143
   }
}
service lmtp {
   unix_listener /var/spool/postfix/private/dovecot-lmtp {
     group = postfix
     mode = 0660
     user = postfix
   }
   user = vmail
}
service managesieve-login {
   inet_listener sieve {
     port = 4190
   }
}
ssl = required
ssl_cert = </etc/letsencrypt/live/mail.domain.tld/fullchain.pem
ssl_cipher_list = 
EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA
ssl_dh_parameters_length = 2048
ssl_key = </etc/letsencrypt/live/mail.domain.tld/privkey.pem
ssl_prefer_server_ciphers = yes
ssl_protocols = !SSLv3
userdb {
   args = /etc/dovecot/dovecot-sql.conf
   driver = sql
}
protocol imap {
   imap_idle_notify_interval = 29 mins
   mail_max_userip_connections = 20
   mail_plugins = " quota imap_quota imap_sieve"
}
protocol lmtp {
   mail_plugins = " sieve"
   postmaster_address = postmaster at domain.tld
}

Thanks in advance.

Chris