RHEL5.2, httpd-2.2.3-11.el5_1.3, tomcat5-5.5.23-0jpp.7.el5_2.1

[Sun Mar 29 04:05:33 2009] [error] ajp_read_header: ajp_ilink_receive
[Sun Mar 29 04:05:33 2009] [error] (120006)APR does not understand this
error code: proxy: read response failed from (null) (localhost)

Googling found a lot of variations of this, and a lot of different
possible explanations that depended on many things. What can I look at
to start to figure out what these messages are complaining about?