If the page loads with 'System error' with cause 'java.lang.NullPointerException at com.atlassian.user.impl.osuser.OSUUserManager.getOpensymphonyUser(OSUUserManager.java:85)' and output logs shows 'user.provider.jdbc.BaseJDBCProvider] init Could not look up DataSource using JNDI location' error, either the Resource for the DataSource is not being loaded by the application server, or it is being loaded but the resource names do not match. Check the names first, so if '/confluence/WEB-INF/classes/osuser.xml' specifies a datasource named 'java:comp/env/jdbc/JiraDS', the datasource specified in server.xml or confluence.xml must be 'jdbc/JiraDS'.
JIRA User Management FAQ