I converted to dovecot. Smooth, no errors, well allmost.
a client used squirrelmail to create dirs and archived his mail with this. After conversion, these are gone, but with a IMAP client, everything is still there.
Squirrelmail is an IMAP client. It all worked for me when I converted. What generally doesn't work is UbeiMiau (webmail), because that's not an IMAP client.
Tell you customer to move his folders under the "INBOX" folder (IMAP syntax will be INBOX.foldername1, INBOX.foldername2 etc) if they aren't. I remember having some problems with that, maybe it's not related but it's worth a try.
I thought of that, but since the O.P. said it was always squirrelmail, I didn't think of posting it. However you're right; it's certainly worth checking.
Looks like this solves it. The directory structure was the problem. The directories where 1 directory to Hi. I logged in with a IMAP client and moved the directories to the INBOX.