[Exim] LMTP delivery error

Using Cyrus imapd 2.1.14 w/ Exim 4.21

When exim attempts to deliver local mail, it coughs up an error in the
mainlog which says:
LMTP error after DATA: 451 4.3.2 cannot create temporary file: File exists

User cyrus has rights to the directory where the socket lives. Is this
an issue with the LMTP configuration, or is something within cyrus hosed?

My exim.conf transports section reads:
local_delivery:
driver = lmtp
command = "/usr/cyrus/bin/deliver -l"
batch_max = 20
user = cyrus

cyrus.conf services section reads:
lmtpunix        cmd="/usr/cyrus/bin/lmtpd"
listen="/var/imap/socket/lmtp" prefork=0

imapd.conf reads (in part):
lmtpsocket: /var/imap/socket/lmtp



Thanks,
-M