You're correct. My conf was bad.

The attached patch to backends.php may help others avoid this in the future.

Addressbooks are not correctly configured in IMP then.

Exactly the same behaviour.

Does autocompletion work when searching by something other than 
non-ASCII characters (e.g. aron)?

In IMP nothing happens when par has been typed in the To box.

When typing par in the attendee box in Kronolith a post is sent. 
Accordning to Firebug the following data is transmitted.

Location with params:
https://myserver/services/imple.php?imple=ContactAutoCompleter%2FimpleApp%3Dkronolith/input=kronolithEventAttendeesreal&kronolithEventAttendeesreal=par

Request headers:
Host: myserver
User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; sv-SE; rv:1.9.2.20) 
Gecko/20110816 Red Hat/3.6.20-2.el6_1 Firefox/3.6.20
Accept: text/javascript, text/html, application/xml, text/xml, */*
Accept-Language: sv,en-us;q=0.7,en;q=0.3
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
X-Requested-With: XMLHttpRequest
X-Prototype-Version: 1.7
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Referer: https://myserver/kronolith/
Content-Length: 31
Cookie: default_horde_view=auto; auth_key=kjsdf789asdhasd; 
imp_key=lasd7f9p8sdfsd; Horde=lkja7s9jhsdjsdfsdvas7; 
horde_menu_expanded=exporganizing
Pragma: no-cache
Cache-Control: no-cache

Response headers:
Date: Sun, 28 Aug 2011 16:27:42 GMT
Server: Apache
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Content-Encoding: gzip
Vary: Accept-Encoding
Content-Length: 110
Connection: close
Content-Type: application/json

Response body:
/*-secure-["\"P\u00e4r Aronsson\" <par.aronsson@telia.com>"]*/

This is nothing specific to your system.

What I meant with using Firebug, is to trace the data sent and 
recieved in the ajax request for the autocompletion. This can watched 
in Firebug's "Network" tab.

I'm in way over my head here, but this is what I've found using Firebug.

In IMP the To box has this code:
<td class="sendtextarea">
<textarea id="to" cols="75" rows="1" name="to" autocomplete="off"></textarea>
<span id="to_loading_img" class="loadingImg" style="display: none;"></span>
</td>
In Kronolith it's this:
<div id="kronolithAttendeesACBox" class="hordeACBox 
kronolithLongField" style="position: relative;">
<ul class="hordeACList">
<li id="kronolithAttendeesACTriggerContainer" class="hordeACListMember">
<input id="kronolithEventAttendeesreal" class="hordeACTrigger" 
name="kronolithEventAttendeesreal" autocomplete="off" style="width: 
100px;">
</li>
</ul>
<span style="float: left; display: inline-block; position: absolute; 
left: -1000px;"></span>
<input id="kronolithEventAttendees" name="attendees" style="display: none;">
</div>

In the case of Kronolith js/prettyautocomplete.js is used. Not in IMP.

Please compare the ajax communication between the browser and the 
server in IMP vs. Kronolith.

For further debugging information, please read 
http://www.horde.org/apps/horde/docs/INSTALL#dynamic-view-troubleshooting

Address autocompletion, in imp, with ldap backend does not work. When 
clicking To: the list is empty until a search is perfomed.

Address autocompletion work as expected in Kronolith.