mod_jk error "cping/cpong after connecting to the backend server failed (errno=24)"

Issue

• The following error in mod_jk logging:

[error] ajp_connect_to_endpoint::jk_ajp_common.c (992): (node22) cping/cpong after connecting to the backend server failed (errno=24)
[error] ajp_send_request::jk_ajp_common.c (1585): (node22) connecting to backend failed. Tomcat is probably not started or is listening on the wrong port (errno=24)

• 503 responses in the httpd access log.