Error when trying to use custom workflow task content type

Hello,

I created a simple project approval workflow which includes one approval task and I deployed it and it worked perfectly. After that, I created a custom workflow approval task (custom content type) to be used instead of "PSWApprovalTask" in the approval task. The custom content type is define as following:

- Elements.xml

<?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <!-- Parent ContentType: Task (0x0108) -->
  <ContentType ID="0x010801002af77dbc5f634cf2be1aa66196284514"               
               Name="Initiative Coordinator"
               Group="Custom Content Types"
               Description="My Content Type"
               Inherits="TRUE"
               Version="0">
    <FieldRefs>
      <FieldRef ID="56C3466D-A725-4AE7-B927-E8DC515C1173" Name="_InitiativeCoordinator"/>      
    </FieldRefs>
    <XmlDocuments>
      <XmlDocument NamespaceURI="http://schemas.microsoft.com/sharepoint/v3/contenttype/forms/url">
        <FormUrls xmlns="http://schemas.microsoft.com/sharepoint/v3/contenttype/forms/url">
          <New>_layouts/M/InitiativeCoordinatorForm.aspx</New>
          <Display>_layouts/M/InitiativeCoordinatorForm.aspx</Display>
          <Edit>_layouts/M/InitiativeCoordinatorForm.aspx</Edit>
        </FormUrls>
      </XmlDocument>
    </XmlDocuments>
  </ContentType>
</Elements>

- Fields.xml

<?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <Field ID="56C3466D-A725-4AE7-B927-E8DC515C1173" 
         Name="_InitiativeCoordinator" DisplayName="Initiative Coordinator" Group="Custom Columns" Type="Text" Required="TRUE"></Field>
</Elements>

After deploying the custyom content type and changing the approval task "ContentTypeId" in the workflow and deploying the workflow, I encountered 2 issues:

• When I go to "Site Settings > Site Columns" I cannot find the custom group that specified in the Fields.xml and also I cannot find my custom field
• When the workflow reaches the workflow approval task generation, it generates the following error "Specified argument was out of the range of valid values.
  Parameter name:
  additionalNodes"

BTW, I'm using OfficeTask activity not CreatTask activity.

What is the wrong with my implementation?