ls -la of the particular Maildir shows:
i.e., no .cur, .new, .tmp but cur, new, tmp.
In those directories are no empty messages at all. Just a lot of seemingly regular messages.
If I use a regular imap client like Thunderbird or Firefox, all works perfectly well.
I have just removed all messages from the 'cur' directory to a temporary location. Now the mailbox seems to be normal. So it seems that there is a particular message which obstructs things...
[30 min later]
I have managed to identify one particular message (from 502) which caused the mail client to freak out. Without this message all is fine.
If someone is interested, I can send the particular file for further examination.