2.1.7 shared folder index issued

Hi,

Sorry to bother the list again so soon after fixing my own problem, but 
I now have this issue when clients try to view shared folders in 
Thunderbird (v12). They can see the shared folder, but the first time 
they click on in nothing happens. The second time they get an 
authentication failure. The third or fourth time it finally loads the 
shared mailbox, and I see this a few times in the logs:

Jun 11 19:57:43 alsace dovecot: imap(sharedviewer <at> integrafin.co.uk): 
Error: mdbox map

/home/indexes/integrafin.co.uk/t/sharedviewer/shared/sharedviewee <at> integrafin.co.uk/storage/dovecot.map.index 
corrupted: U
nexpectedly lost shared/viewee <at> integrafin.co.uk/INBOX uid=73129 
map_uid=74192
Jun 11 19:57:43 alsace dovecot: imap(sharedviewer <at> integrafin.co.uk): 
Disconnected: Internal error occurred. Refer to server log for more 
information. [2012-06-11 19:57:43] in=308 out=820
Jun 11 19:57:43 alsace dovecot: auth: Debug: auth client connected 
(pid=1957)

Ideally I'd like shared mailboxes to work in the first click - any ideas?

Cheers

Alex