How to find missing web part

Figured it out.

The error message is the one from the .webpart file:

<?xml version="1.0" encoding="utf-8"?>
<webParts>
  <webPart xmlns="http://schemas.microsoft.com/WebPart/v3">
    <metaData>
      <!--
      The following Guid is used as a reference to the web part class, 
      and it will be automatically replaced with actual type name at deployment time.
      -->
      <type name="7F8C4D34-6311-4f22-87B4-A221FA8735BA" />
      <importErrorMessage>Cannot import Project Filter.</importErrorMessage>
    </metaData>
    <data>
      <properties>
        <property name="Title" type="string">Project Filter</property>
        <property name="Description" type="string">Provides a list of Projects that can be used to Filter other Web Parts.</property>
      </properties>
    </data>
  </webPart>
</webParts>

The problem is that the original .webpart file was created on a 32-bit system with Visual Studio Extensions for WSS installed.

However as I'm now on a 64-bit machine VSEWSS is unavailable, and I believe that results in the above GUID not being substituted as I am not using those deployment tools.

Replacing the GUID with the full type name works.

So if you encounter the error message from your importErrorMessage node, then check that your type node in the .webpart file looks more like this (unrelated example):

<type name="TitleWP.TitleWP, TitleWP, Version=1.0.0.0, Culture=neutral, PublicKeyToken=9f4da00116c38ec5" />

This is in the format: Class, Namespace, Version, Culture, PublicKey

You can grab that easily from the web.config file associated with your SharePoint instance, as it will be in the safe controls list.

In all the times I have come across an error in changing from 64bit to 32bit, the change in the number of bits is usually a red herring and the actual cause of the failure is something else.

Check the sharepoint logs for the WebPart errors. They will help show exactly what is wrong. Post the relevant errors if you still need help.