[imp] Problems installing imp

[cyx]

At the moment I try to install a new horde-server as backup. 

Installation went well, but now when I open imp or kronolith, I see "error
communicating with the server" error box.

Backends.local.php is the same as the one working on our prime horde server.

In http-error.log I see: [error] [client x.x.x.x] File does not exist:
/usr/local/www/apache22/data/horde/services/ajax.php/horde/topbarUpdate,
referer: https://webmailneu.htlwrn.ac.at/imp/dynamic.php?page=mailbox

 

Any ideas?

Regards

Werner