I recently upgraded a well working Ubuntu server instance from 8.04 LTS to 10.04 LTS as the support for 8.04 LTS will end this April 2013. This server system uses a JBoss(+Tomcat)/Apache2.2 stack for delivering its services. The connection between Apache and JBoss has been realised using the mod_jk plugin for Apache which uses the AJP approach. In 8.04, this worked fine. After having performed the do-release-upgrade, this connection did not work anymore. Increasing the logging level of the mod_jk plugin to debug, the logfile contained "missing uri map for <servername>:<path>" messages.
Looking around for possible causes, I found this email conversation, suggesting to use the JkMountCopy All directive in the global Apache2 configuration file. This solved the problem.
Another result suggested even not using mod_jk anymore but using the newer mod_proxy_ajp.