dovecot: auth-worker ... pam_start() failed: Critical error - immediate abort

This has been working fine with no system changes for some time now. I have googled and found advice apparently for prior versions of dovecot and different OSes. It is not even temporarily fixed by system reboots.

# cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=14.04
DISTRIB_CODENAME=trusty
DISTRIB_DESCRIPTION="Ubuntu 14.04.4 LTS"

# dovecot --version
2.2.9

# telnet 127.0.0.1 110
Trying 127.0.0.1...
Coected to 127.0.0.1.
Escape character is '^]'.
+OK Dovecot (Ubuntu) ready.
user ********
+OK
pass ********
-ERR [SYS/TEMP] Temporary authentication failure. [****:2016-04-18 17:21:23]
quit
+OK Logging out
Coection closed by foreign host.

mail.log:

Apr 18 10:21:21 **** dovecot: auth-worker(1891): Error: pam(********,127.0.0.1): pam_start() failed: Critical error - immediate abort
Apr 18 10:21:27 **** dovecot: pop3-login: Aborted login (auth failed, 1 attempts in 6 secs): user=<********>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, secured, session=<osTalsUwpAB/AAAB>

syslog:

Apr 18 10:21:21 **** t of memory [1891]
Apr 18 10:21:21 **** dovecot: auth-worker(1891): Error: pam(********,127.0.0.1): pam_start() failed: Critical error - immediate abort
Apr 18 10:21:27 **** dovecot: pop3-login: Aborted login (auth failed, 1 attempts in 6 secs): user=<********>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, secured, session=<osTalsUwpAB/AAAB>

ke.log:

Apr 18 10:21:21 **** t of memory [1891]

mail.err:

Apr 18 10:21:21 **** dovecot: auth-worker(1891): Error: pam(********,127.0.0.1): pam_start() failed: Critical error - immediate abort

# dovecot -n
# 2.2.9: /etc/dovecot/dovecot.conf
# OS: Linux 3.19.0-58-generic x86_64 Ubuntu 14.04.4 LTS
disable_plaintext_auth = no
mail_location = mbox:~/mail:INBOX=/var/mail/%u
mail_privileged_group = mail
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
}
protocols = " imap pop3"
ssl_cert = </etc/dovecot/dovecot.pem
ssl_key = </etc/dovecot/private/dovecot.pem
userdb {
driver = passwd
}

Others, from the googling, have suggested increasing auth_worker_max_request_count. However, that appears to be for earlier versions of dovecot and does not seem to have been universally successful anyway.

Oh, and here is meminfo, since the 't of memory' error seems to suggets and out of memory condition, though that would be odd immediately after a reboot.

# cat /proc/meminfo
MemTotal: 15274824 kB
MemFree: 14900584 kB
MemAvailable: 14904420 kB
Buffers: 21736 kB
Cached: 139484 kB
SwapCached: 0 kB
Active: 166408 kB
Inactive: 91148 kB
Active(anon): 96676 kB
Inactive(anon): 6108 kB
Active(file): 69732 kB
Inactive(file): 85040 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 15666172 kB
SwapFree: 15666172 kB
Dirty: 0 kB
Writeback: 0 kB
AnonPages: 96380 kB
Mapped: 52136 kB
Shmem: 6452 kB
Slab: 39656 kB
SReclaimable: 21832 kB
SUnreclaim: 17824 kB
KeelStack: 2880 kB
PageTables: 9588 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 23303584 kB
Committed_AS: 622192 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 117704 kB
VmallocChunk: 34359614368 kB
HardwareCorrupted: 0 kB
AnonHugePages: 45056 kB
CmaTotal: 0 kB
CmaFree: 0 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 96720 kB
DirectMap2M: 1939456 kB
DirectMap1G: 14680064 kB

Any ideas or other things to check? I may have to shut down the postfix services and move the mbox contents over to the other server temporarily. It'll just throw coection erros on all of the clients until I get it resolved.

Thanks, in advance, for any help in this matter.