I've got a DAV resource with both CalDAV and CardDAV collections (both my own and shared ones from other users). When opening kaddressbook and trying to access a shared contacts list, the akonadi-tray give me the following error messagge (in German, I'm afraid): Der Server hat ungültige Daten gesendet" (something like "server has sent invalid data"). When opening akonadiconsole and selecting this specific resource, it gives the error "Can't fetch item from backend".
Both my own contact list and calendars (own and shared ones) seem to work fine.
Some of the errror messages in akonadiconsole (debugger) were:
May-2012 12:35:15 +0000" REMOTEREVISION "\"289063139-271\"" FLAGS () ANCESTORS ((34 "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/") (27 "akonadi_davgroupware_resource_0") (0 ""))) 
522 OK UID FETCH completed 
523 UID REMOVE 16517 
523 OK REMOVE complete 
524 UID FETCH 16518 CACHEONLY ANCESTORS INF EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME) 
* 16518 FETCH (UID 16518 REV 0 REMOTEID "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/cgpitem284.vcf" MIMETYPE "text/directory" COLLECTIONID 34 SIZE 0 DATETIME "15-May-2012 12:35:15 +0000" REMOTEREVISION "\"289063139-284\"" FLAGS () ANCESTORS ((34 "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/") (27 "akonadi_davgroupware_resource_0") (0 ""))) 
524 OK UID FETCH completed 
525 UID REMOVE 16518 
525 OK REMOVE complete 
526 UID FETCH 16519 CACHEONLY ANCESTORS INF EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME) 
* 16519 FETCH (UID 16519 REV 0 REMOTEID "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/cgpitem290.vcf" MIMETYPE "text/directory" COLLECTIONID 34 SIZE 0 DATETIME "15-May-2012 12:35:15 +0000" REMOTEREVISION "\"289063139-290\"" FLAGS () ANCESTORS ((34 "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/") (27 "akonadi_davgroupware_resource_0") (0 ""))) 
526 OK UID FETCH completed 
527 UID REMOVE 16519 
527 OK REMOVE complete 
528 UID FETCH 16520 CACHEONLY ANCESTORS INF EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME) 
* 16520 FETCH (UID 16520 REV 0 REMOTEID "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/cgpitem313.vcf" MIMETYPE "text/directory" COLLECTIONID 34 SIZE 0 DATETIME "15-May-2012 12:35:15 +0000" REMOTEREVISION "\"289063139-313\"" FLAGS () ANCESTORS ((34 "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/") (27 "akonadi_davgroupware_resource_0") (0 ""))) 
528 OK UID FETCH completed 
529 UID REMOVE 16520 
529 OK REMOVE complete 
530 UID FETCH 16521 CACHEONLY ANCESTORS INF EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME) 
* 16521 FETCH (UID 16521 REV 0 REMOTEID "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/cgpitem332.vcf" MIMETYPE "text/directory" COLLECTIONID 34 SIZE 0 DATETIME "15-May-2012 12:35:15 +0000" REMOTEREVISION "\"289063139-332\"" FLAGS () ANCESTORS ((34 "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/") (27 "akonadi_davgroupware_resource_0") (0 ""))) 
530 OK UID FETCH completed 

or
222 UID FETCH 16537 FULLPAYLOAD EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME) 
222 NO Unable to fetch item from backend 
223 STATUS 34 (MESSAGES UNSEEN SIZE) 
* STATUS "akonadi_davgroupware_resource_0/[pr_austria@wycliffe.org]Contacts" (MESSAGES 135 UNSEEN 135 SIZE 0) 
223 OK STATUS completed 
224 UID FETCH 16538 FULLPAYLOAD EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME) 
224 NO Unable to fetch item from backend 
225 STATUS 34 (MESSAGES UNSEEN SIZE) 
* STATUS "akonadi_davgroupware_resource_0/[pr_austria@wycliffe.org]Contacts" (MESSAGES 134 UNSEEN 134 SIZE 0) 
225 OK STATUS completed 
226 UID FETCH 16539 FULLPAYLOAD EXTERNALPAYLOAD (UID REMOTEID REMOTEREVISION COLLECTIONID FLAGS SIZE DATETIME) 

in the agent browser the resource shows less and less entries (dropping slowly from about 200 to around 100 right now), but when browsing through the web interface, no changes are seen (which I'm glad about otherwise I would loose data...)


.xsession-errors shows:
someone else requested item 16631 as well, marking as processed 
someone else requested item 16631 as well, marking as processed 
continuing 
request for item 16631 "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/cgpitem121.vcf" failed: "Resource was unable to deliver item" 
ItemRetrieverException :  Resource was unable to deliver item
continuing 
request for item 16631 "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/cgpitem121.vcf" failed: "Resource was unable to deliver item" 
ItemRetrieverException :  Resource was unable to deliver item
continuing 
request for item 16631 "https://mail.jaars.org/CardDAV/~pr_austria@wycliffe.org/Contacts/cgpitem121.vcf" failed: "Resource was unable to deliver item" 
ItemRetrieverException :  Resource was unable to deliver item
posting retrieval request for item 16632  there are  1  queues and  0  items in mine 
request for item 16632 still pending - waiting 
processing retrieval request for item 16632  parts: ("RFC822")  of resource: "akonadi_davgroupware_resource_0" 
posting retrieval request for item 16632  there are  1  queues and  0  items in mine 
request for item 16632 still pending - waiting 
posting retrieval request for item 16632  there are  1  queues and  1  items in mine 
request for item 16632 still pending - waiting 
Setting new source 
New source:  QUrl( "file:///usr/share/sounds/KDE-Sys-App-Error-Serious.ogg" )  
Transitioning to state "playing" 
kmix(13401) sink_input_cb: Ignoring sink-input due to it being designated as an event and thus handled by the Event slider 

Any more information I can provide to help fix this bug?