I observe that in function read_socket_line() in line ckpool.c:455 the recv() function returns 0 when 'validateaddress' is called after 'getblocktemplate' returned a very large response over the socket.

Following manpage, a return value of 0 for recv() has to be interpreted as 'peer has performed an orderly shutdown' which is not handled in read_socket_line(). Question remains why bitcoind is closing the socket, but at least this has to be handled accordingly by ckpool.