[horde] Problem to create folder after update horde from 3.0.5 and imp 4.0.3

[Mailinglist]

Hallo

we have running horde 3.0.5 with imp 4.0.3. this works fine. now we
upgrade to horde 3.1.1  and IMP  4.1.1. After the upgrade we have a lot
of problems with folders.
We use imp with IMAP, the IMAP server is a Courier IMAP. In the IMP
4.0.3 all folders are subfolders of the inbox folder, but in the new IMP
4.1.1 they are no more subfolder.
In IMP 4.0.3 the folders look like

- INBOX
   -  Trash
   -  sent-mail
   -  Folder1

And in IMP 4.1.1 the folders look like

- INBOX
- Trash
- sent-mail
- Folder1

If i want to create a folder under bei Inbox i get the message
"The folder "INBOXtrash" was not created. This is what the server said:
Invalid mailbox name."

If i go under options -> Server and Folder Information
<https://webmail.ml-networld.de/horde/services/prefs.php?group=server&app=imp> 

and want to create a trashfolder i get this messagse
*"Warning*: imap_createmailbox(): supplied argument is not a valid imap
resource in */pathtowebmail/webmail/horde/imp/lib/Folder.php* on line *298"*

Is this a configuration problem on our site?
We use the same servers.php as under imp 4.0.3

thx

kai