Sieve: Invalid/unknown path to storage

[Kay-Michael Voit]

Hi, I have just updated from Ubuntu 15.04 to 15.10. Unfortunately, sieve 
scripts are not recognized anymore.

Here is my version:
dovecot --version
2.2.18
apt-cache policy dovecot-sieve
dovecot-sieve:
  Installed: 1:2.2.18-2ubuntu2
  Candidate: 1:2.2.18-2ubuntu2
  Version table:
 *** 1:2.2.18-2ubuntu2 0
        500 http://us.archive.ubuntu.com/ubuntu/ wily/main amd64 Packages
        100 /var/lib/dpkg/status

And this is the log entry:
Dec 25 09:24:12 home-le dovecot: lda(user1): Warning: sieve: file 
storage: Active sieve script symlink /home/user1/.dovecot.sieve is 
broken: Invalid/unknown path to storage (points to /home/user1/sieve).
Dec 25 09:24:12 home-le dovecot: lda(user1): 
msgid=<567cfd2c.7070...@example.org>: saved mail to INBOX
Dec 25 09:24:12 home-le postfix/local[28536]: 70876BD035A: 
to=<us...@example.org>, relay=local, delay=0.07, 
delays=0.02/0.01/0/0.04, dsn=2.0.0, status=sent (delivered to command: 
/usr/lib/dovecot/deliver -c /etc/dovecot/dovecot.conf -m "${EXTENSION}")

It is not a permission issue (though there should even be one with more 
restrictive permissions)
otheruser@home-le:~$ cat /home/user1/.dovecot.sieve
require ["variables","date", "include"];

include :personal "00-delete";
include :personal "01-spam";
include :personal "50-bulk_filter";

It is apparently also not a path normalization issue (also according to 
the correct "points to" path in the log). I used an absolute path in the 
symlink:
user1@home-le:~$ ls -la .dovecot.sieve
lrwxrwxrwx 1 user1 user1 33 Dec 25 09:23 .dovecot.sieve -> 
/home/kay/sieve/managesieve.sieve

Any idea what could be wrong here?

Here is my dovecot -n output:
namespace inbox {
  inbox = yes
  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 =
  separator = .
  type = private
}
passdb {
  driver = pam
}
plugin {
  sieve = file:~/.dovecot.sieve
  sieve_dir = ~/sieve
}
postmaster_address = postmas...@example.net
protocols = " imap sieve"
service managesieve-login {
  inet_listener sieve {
    port = 4190
  }
}
ssl_ca = </etc/ssl/certs/startssl.ca.pem
ssl_cert = </etc/ssl/certs/net.example.home-le.pem
ssl_key = </etc/ssl/private/net.example.home-le.key
submission_host = 192.168.153.1
userdb {
  driver = passwd
}
protocol lda {
  mail_plugins = " sieve"
}
protocol imap {
  mail_max_userip_connections = 50
}