So I have to edit it using the RAW tab?
https://www.dropbox.com/s/gabakc75wmi0x5m/Screensh...
?

Just added a second account, rebooted the server and now its broken again ?!
The main exim log says:
2014-07-08 07:27:10 plain authenticator failed for (daslichts-pro.multi.box) [<<snip>>]: 435 Unable to authenticate at present (set_id=<<snip>>): failed to connect to socket /var/run/courier/authdaemon/socket: Permission denied
...
root@h2318011:~# ls -l /var/run/courier/authdaemon/socket
srwxrwxrwx 1 root root 0 Jul 8 07:23 /var/run/courier/authdaemon/socket

[EDIT]
The /var/run/courier/authdaemon had the wrong permissions.

This helped me:
https://lists.debian.org/debian-user/2006/12/msg00636.html

Hello,
I am trying to access a IMAP Mailbox with apple Mail, but when I try to connect I get :
"The server returned the error: Unable to open this mailbox."

Any Ideas what could be wrong, please?

In connection doctor I see that teh coinnection was successfull
The exim main log says :
input sent without waiting for greeting

[Fixed] by sending myself an email.

Is there meanwhile a Webmailer available , please ?