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?

Thanks!

//per