[exim] what is this "Failed to get write lock for"

Hi. I have noticed in logs :

2006-05-16 09:27:46 1Fft0D-0002em-Kr Failed to get write lock for
/var/spool/exim4/db/name_of_the_transport.lockfile: timed out
2006-05-16 09:27:46 1FdqWa-0004ml-4i Failed to get write lock for
/var/spool/exim4/db/name_of_the_transport.lockfile: timed out

I use exim 4.50-8 from debian distribution with cyrus-imap2.2.

Here is configuration of the mentioned transport:

name_of_the_transport:
        debug_print = "T: name_of_the_transport for $local_part@$domain"
        driver = smtp
        hosts_avoid_tls = cyrus_lmtp.server
        protocol = lmtp
        hosts_try_auth = *
        hosts_require_auth = cyrus_lmtp.server
        hosts = cyrus_lmtp.server
        return_path_add = false
        hosts_override = true
        envelope_to_add = true
        delivery_date_add = true

During this messages lmtp service is in good state and accept everything.