When a filename property is specified, LogbackValve searches for the config by prepending catalina.base and catalina.home to the path, then looks for it as a resource. Neither of those manages to find a file referenced with an absolute path (e.g. /tmp/logback-access.xml).

A simple workaround is to create a symlink from the default location to the external file, but this shouldn't be necessary. Specifying a file location outside tomcat home is convenient in certain situations and should be supported.