Dovecot: folder appears empty

NILToday I encountered an unknown issue to me with my Imap server

dovecot. In roundcube mail web client, my Inbox folder appeared empty

after being reading a mail. My Android mail client K9-Mail was

displaying "IOException:readStringUnti...." when trying to synchronize

this folder.

I solved it easily by connecting to my server with SSH, cd-ing into

the maildir directory and in the Inbox folder, renamed

if it fix the problem).

And now, mails are back. This is the very first time I have a problem

of this kind with dovecot...