After upgrading Activiti 5.8 to 5.9 you get an exception when trying to post a comment (in the top right corner) for any task.
This only seems to happen when using a PostgreSQL database. When using H2 this will not happen. I tried both, vanilla.
The Exception (After trying to comment "Hello World"):
Check the attachment for a full stacktrace!
Debian VM, Tomcat 7, PostgreSQL 8.4