Out of laziness, I reused an existing Tomcat instance that had OpenAM 12 installed and configured.
Stop Tomcat; Remove OpenAM 12 war file and the exploded OpenAM 12 directory; Deploy OpenAM 11 was file; Start Tomcat.
Bomb! As soon as Tomcat started, I encountered HTTP Status 500 error.
Took me a while to realize that I have not yet cleaned up the hidden OpenAM configuration.
[ec2@am0 ~]$ cd .openamcfg/
[ec2@am0 .openamcfg]$ ls -altr
-rw-rw-r--. 1 ec2:ec2 25 Mar 1 18:55 AMConfig_data_opt_am1200_webapps_auth_
This file pointed to a configuration directory where OpenAM 12 was previously installed. Now that OpenAM 11 was installed, the above AMSetupFilter.doFilter error appeared.
[ec2@am0 .openamcfg]$ rm -fr AMConfig_data_opt_am1200_webapps_auth_
Start Tomcat again.