Problem with simple h:form, h:selectManyListbox and h:commandButton

I'm currently having trouble with this simple form getting working:
<h:form id="choose_projects_form">
<div class="table_big">
<div class="table_header">
<h:outputText value="#{msg.USER_REPORTS_CHOOSE_PROJECTS_TITLE}" />

<div class="para">
<div class="form_label">
<h:outputLabel for="selectedProjects" value="#{msg.USER_CHOOSE_PROJECTS}" />

<div class="form_field">
<h:selectManyListbox styleClass="projects" id="selectedProjects" size="5" value="#{choosenProjectsReportsController.selectedProjects}" required="true" requiredMessage="#{msg.USER_FIELD_SELECTED_PROJECT_REQUIRED}">
<f:converter converterId="ProjectConverter" />
<f:selectItems value="#{choosenProjectsReportsController.availableProjects}" var="project" itemValue="#{project}" itemLabel="#{msg[project.i18nKey]}" />

<div class="clear"></div>

<h:message for="selectedProjects" errorClass="errors" fatalClass="errors" warnClass="errors" showSummary="true" />

<div class="para notice">
<h:outputText value="#{msg.USER_CHOOSE_PROJECTS_NOTICE}" />

<div class="table_footer">
<h:commandButton styleClass="reset" type="reset" value="#{msg.BUTTON_RESET_FORM}" />
<h:commandButton styleClass="submit" type="submit" action="#{choosenProjectsReportsController.doChooseProjects()}" value="#{msg.BUTTON_SHOW_CHOOSEN_PROJECT_STATISTICS}" />

The backing bean (web controller) has this defined:

import javax.enterprise.context.SessionScoped;
@Named ("choosenProjectsReportsController")
public class ReportsChoosenCustomersWebSessionBean extends BaseReportsController implements ReportsChoosenCustomersWebSessionController {
private final List<ReportProject> availableProjects; // +getter as no setter is needed/possible
private ReportProject selectedProjects[]; // + getter/setter

In constructor the list is initialized, in a @PostConstruct method it is being filed (ReportProject is an enumeration).

All projects are being shown an the selectManyListbox and commandButton are both correctly being rendered. If it select one (or more) and hit the submit button, a POST request goes.

The problem is that doChooseProjects() is not being invoked and also the converter's getAsObject() method is not invoked. Also no error message shows up (including log file).

A search on Internet like "selectManyListbox converter action not invoked" (no quotes) doesn't turn up any useful.
javaee (x) jsf (x)
cc !java nobody can help here? :-(