Version:           4.4.10 (using KDE 4.6.0) 
OS:                Linux

I upgraded to KDE 4.6.0 this week, had  been on KDE 4.4 and had not seen the problem. Even after the upgrade all was well until last night, when I start getting the error when trying to read a newsgroup - the nntp server responds correctly to Thunderbird, so it is a KDE problem, not a server problem. The error is 
QUOTE
     Internal Error
     Please send a full bug report at http://bugs.kde.org
     Unexpected server response to XOVER command:
     Subject:
UNQUOTE
I tried deleting both ~/.kde4/share/config/kontact_summaryrc and ~/.kde4/share/config/kontactrc but the bug remains.
I also tried deleting the newsgroup and reloading it. It successfully retrieves the topic groups but cannot read their contents. 

Reproducible: Always

Steps to Reproduce:
Open Kontact, select Usenet, click on the newsgroup.

Actual Results:  
QUOTE
     Internal Error
     Please send a full bug report at http://bugs.kde.org
     Unexpected server response to XOVER command:
     Subject:
UNQUOTE

Expected Results:  
It should open the newsgroup and show unread articles.

The server responds correctly in Thunderbird.

The group is a closed (private) group (nlzero.com) and I have no other NNTP accounts so cannot test against other servers.