The doc page Connecting JIRA to DB2 should have a note informing users that the database user needs to have assigned a default schema.

Also, if the db user has a default db schema associated with him, I would imagine that for most users, there isn't a need to specify the schema in enitityengine.xml .

...
<datasource name="defaultDS" field-type-name="db2"
      schema-name="DB2INST1"
      helper-class="org.ofbiz.core.entity.GenericHelperDAO"
...