I also tried with AM version 6.5.4 with the same java, OpenDS version as above and it worked. Hence, I feel there is a bug with the new version of AM 6.5.5.
Precisely below is my test environment where 6.5.5 is giving error and 6.5.4 is working=> tomcat 9.0.34.0, Oracle Java 11.0.17+10-LTS-269,