Socket buffer used for communication between Frontend and Server is too small for all of the cases. For example, testing preprocessing steps with large data inputs or many preprocessing steps can cause the following error:

Size of the response received from Zabbix server "localhost" exceeds the allowed size of 1048576 bytes. This value can be increased in the ZBX_SOCKET_BYTES_LIMIT constant in include/defines.inc.php.

Buffer should be increased (sasha proposed to raise buffer size to 16 megs).