intermittant passdb error

[NeuralNET]

I'm having great difficulty debugging this problem.. I'm having 
intermittant problems connecting to dovecot. This started a couple of 
days ago following an apt-get update on the server.

I'm running Ubuntu 14.04, dovecot 2.2.9. Here's my dovecot -n
---
# 2.2.9: /etc/dovecot/dovecot.conf
# OS: Linux 3.19.0-59-generic x86_64 Ubuntu 14.04.4 LTS
auth_mechanisms = plain login
mail_location = maildir:~/Maildir
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 =
}
passdb {
   driver = pam
}
plugin {
   sieve_dir = ~/sieve
}
protocols = imap
service auth {
   unix_listener /var/spool/postfix/private/auth {
     mode = 0666
   }
   user = $default_internal_user
}
ssl = required
ssl_cert = </etc/dovecot/box.neuralnet.org.uk_crt.pem
ssl_key = </etc/dovecot/box.neuralnet.org.uk_key.pem
ssl_protocols = !SSLv2 !SSLv3
userdb {
   driver = passwd
}
---

Pretty standard stuff. In addition disable_plaintext_auth = yes is set 
in 10-auth.conf

This showed up in the mail.err after the reboot;

---
May 17 09:04:51 server dovecot: imap-login: Error: read(anvil) failed: EOF
May 17 09:04:51 server dovecot: auth: Error: read(anvil-auth-penalty) 
failed: EOF
May 17 09:04:51 server dovecot: auth: Error: 
net_connect_unix(anvil-auth-penalty) failed: Permission denied
---

At that point

The following commands were run on the server a few minutes apart with 
no changes made in the interim;

---
username at box /etc/dovecot $ doveadm user username
field    valueuserdb lookup: user username doesn't exist

username at box /etc/dovecot $ doveadm user username
field    value
uid    1000
gid    1000
home    /home/username
mail    maildir:~/Maildir
system_groups_user    username
---

so, passdb wasn't accessible, but now it is ?

Imap logins are showing in mail.log as failing, but thunderbird is 
currently connected;

---
May 18 12:00:58 box dovecot: imap-login: Disconnected (auth failed, 3 
attempts in 19 secs): user=<username>, method=PLAIN, rip=10.0.0.3, 
lip=10.0.0.99, TLS, session=<********>
May 18 12:08:27 box dovecot: imap-login: Disconnected (auth failed, 3 
attempts in 20 secs): user=<username>, method=PLAIN, rip=10.0.0.3, 
lip=10.0.0.99, TLS, session=<********>
May 18 12:08:50 box dovecot: imap-login: Disconnected (auth failed, 3 
attempts in 12 secs): user=<username>, method=PLAIN, rip=10.0.0.3, 
lip=10.0.0.99, TLS, session=<********>
---

do I conclude that method=plain is failing, whereas method=login 
succeeds ? I tried setting auth_debug_password=yes and get the following;

---
May 18 12:26:42 box dovecot: auth: Debug: client in: 
CONT#0111#011A********== (previous base64 data may contain sensitive data)
May 18 12:26:42 box dovecot: auth-worker(12214): Debug: Loading modules 
from directory: /usr/lib/dovecot/modules/auth
May 18 12:26:42 box dovecot: auth-worker(12214): Debug: 
pam(expert,10.0.0.3): lookup service=dovecot
May 18 12:26:42 box dovecot: auth-worker(12214): Debug: 
pam(expert,10.0.0.3): #1/1 style=1 msg=Password:
May 18 12:26:42 box dovecot: auth-worker(12211): pam(username,10.0.0.3): 
pam_authenticate() failed: Authentication failure (password mismatch?) 
(given password: ******)
May 18 12:26:44 box dovecot: auth: Debug: client passdb out: 
FAIL#0112#011user=username
---

Any assistance debugging this further is greatly recieved! Thanks.