Initially, having the following error:

2008-11-12 18:24:31,103 TP-Processor34 WARN [org.openid4java.server.RealmVerifier] RP discovery / realm validation disabled; this option SHOULD be enabled for OPs

In order to fix it, the following modification to the serverManager Spring configuration (atlassian-crowd-1.5.2/crowd-openidserver-webapp/WEB-INF/classes/applicationContext-OpenIDServer.xml) should create a RealmVerifier with RP Discovery enabled.

    <bean id="serverManager" class="org.openid4java.server.ServerManager">
        <property name="privateAssociations" ref="associationStore"/>
        <property name="sharedAssociations" ref="associationStore"/>
        <!-- edit this to reflect the URL of the OpenID server: -->
        <property name="OPEndpointUrl" value="http://localhost:8087/openidserver/server"/>
        <property name="userSetupUrl" value="http://localhost:8087/openidserver/server"/>
        <property name="realmVerifier">
            <bean class="org.openid4java.server.RealmVerifier"/>
        </property>
    </bean>

After performing the change above, the following error is displayed: "Unable to verify the specified OpenID: failed to perform curly request [couldn't connect]".

The log file shows the following errors:
2008-12-12 12:50:05,089 TP-Processor49 WARN [org.openid4java.message.AuthRequest] openid.trust_root should not be present in OpenID2 auth requests.
2008-12-12 12:50:05,896 TP-Processor49 WARN [org.openid4java.message.AuthRequest] openid.trust_root should not be present in OpenID2 auth requests.