[Dovecot] Lost transaction log file in alpha2

I had run alpha1 pretty uneventfully for several weeks. I upgraded to
alpha2 a couple of days ago and have since suffered two incidents of
"Lost transaction log", resulting in the client being unable to
access the mailbox until I "rm dovecot.index.log*"

Sep 14 10:48:35 caddy dovecot: imap(jberry): Lost transaction log
file /Users/jberry/Maildir/dovecot.index.log seq 1

I should point out that this is a server with a single imap user
(myself) and that I know only one imap client was trying to access
the server at that point in time (though the client sw Mail.app may
have had multiple connections to the server).

Any suggestions/ideas/thoughts on whether something might have
changed in alpha2 to start causing this?

James.