inaccessible mailbox for one user

[Lists Nethead]

> Hi,
>
> A user clicks on a mail in Horde IMP and next the inbox appears  
> completely empty. This message files all still there of course, but  
> the problem is that I cannot run any cyrus command on that mailbox,  
> reconstruct, chk_cyrus or anything.
>
> Is there a lock possibly sitting somewhere that prohibits running  
> any commands? Any other hints as to where I should start?
>

More:

Cyrus is 3.0.8

I suspected that the problem of not being able to run any utilities  
was not only a Cyrus problem, su I restarted the virtual server  
(FreeBSD jail), then the locking was gone. Fine, so I ran the usual  
reconstruct.

Now one can see the messages that arrived *after* the crash, but none  
of the ones arriving before (>6200 in inbox and plenty of subfolders).

It is just one account, the others work ok.

Where then is the problem? If reconstruct cannot fix it, what can?