Service Provider plugin fails to load required interfaces

I have written a Java plugin for RTC 4.0.6 which is an extension to com.ibm.team.repository.service.serviceProvider.
I have also declared 2 prerequisite interfaces for this plugin.
Everything looks ok, apparently there are no syntax errors, and the plugin is build correctly.
Anyway, at runtime, the 2 required interfaces are not loaded, and I receive en exception that prevents the plugin from running.
It seems that the application server fails to parse the plugin configuration.
These are the contents of the plugin.xml:

<?xml version="1.0" encoding="UTF-8"?>
<?eclipse version="3.4"?>
<plugin>
    <extension point="com.ibm.team.repository.service.serviceProvider">
        <serviceProvider
            componentId="com.ibm.team.mycustomer"
            implementationClass="com.ibm.team.mycustomer.service.PluginUtility">           
            <provides>
                <providedService interface="com.ibm.team.mycustomer.common.IPluginUtility"/>
            </provides>
            <prerequisites>
              <requiredService interface="com.ibm.team.repository.service.IRepositoryItemService"/>
              <requiredService interface="com.ibm.team.workitem.service.IWorkItemServer"/>
            </prerequisites>
        </serviceProvider>
    </extension>
</plugin>

Any guess?