Allan, before you started getting these errors, had you either upgraded your ACS installation from a previous release? OR exported and reimported the data from your database? There had been some oddness about the referential integrity constraints in the security stuff. A constraint defined twice. When initially installed it runs without error but export/import causes the constraints to be reenabled in the opposite order and that throws errors.
You can see what constraints are enabled on the table with:
select c.constraint_name, search_condition, delete_rule,
column_name, position
from user_constraints c, user_cons_columns cc
where c.constraint_name = cc.constraint_name
and c.owner = cc.owner
and c.table_name = 'SEC_SESSIONS';
