[Dovecot] Problems with (probably) Dovecot

[Frank Neps]

Hello altogether,

I've got Dovecot 0.99 installed on a Suse Linux 9.2 system, together 
with Postfix. I need to upgrade that server in the near future to new 
hardware and new versions, but at the moment it would be great, if I 
could find the solution for the following problem without upgrading. 
Unfortunately I didn't find a solution in Google.

The system has enough memory and disc space, it was running for about 4 
or 5 years without problems now.

Trying to access my messages on the server via IMAP protocol using 
Outlook (2000 or 2002 or 2003) leads to an error message "Invalid 
messageset: xxxxxxxx"  (where xxxxxxxx is a  changing number). No data 
is displayed, the headers are not updated. Squirrelmail says, that the 
messages are not available any more. And Thunderbird joins in with 
Outlook and also tells me something about an invalid messageset. For 
some reasons (and some messages, not all) Kontact / Kmail does not give 
me error message, however, my main system is not a Linux one and 
therefore I'd like to get that fixed, if possible.

Somewhere in Google I found a note about IMAP index files, which may be 
corrupt and need to be rebuilt, but I didn't find any Index files, I 
must admit. 

Any idea how to solve that?

Thanks in advance and best regards,

Frank