I recently (as in 5 minutes ago) ran into this error and thought I would share since it cost me a good hour of work today. I have a WebPart that I need to add another assembly to. The second assembly was responsible for getting data from Active Directory. So I went through all the hoops of adding the assembly

• Strong name the assembly and added a reference to my WebPart project.
• Made an entry into the manifest.xml file
• Made an entry into the wsp_structure file.

Now that should be it but when I tried to add the WebPart I now got an error that said Invalid WebPart Tag. What the hell does that mean? The log didn't provide much more additional information. Eventually I noticed a mistake in my manifest.xml file. Take a look at the before and after.

Here was my original manifest.xml file.

<WebPartManifest xmlns="http://schemas.microsoft.com/WebPart/v2/Manifest">

  <Assemblies>

    <Assembly DeploymentTarget="GlobalAssemblyCache" FileName="Infusion.WebParts.dll">

      <SafeControls>

        <SafeControl Assembly="Infusion.WebParts, Version=1.0.0.0, Culture=neutral, PublicKeyToken=434425b09396c611" Namespace="Infusion.WebParts" TypeName="*" Safe="True" />

      </SafeControls>

    </Assembly>

</Assemblies>

<Assemblies>

    <Assembly DeploymentTarget="GlobalAssemblyCache" FileName="Infusion.DirectoryServicesHelper.dll">

      <SafeControls>

        <SafeControl Assembly="Infusion.DirectoryServicesHelper.dll, Version=1.0.3058.16424, Culture=neutral, PublicKeyToken=3e458cd467f1ce4f" Namespace="Infusion.DirectoryServicesHelper" TypeName="*" Safe="True" />

      </SafeControls>

    </Assembly>

 </Assemblies>

  <DwpFiles>

    <DwpFile FileName="Infusion.WebParts.DisplayUsersFromADGroup.dwp" />

  </DwpFiles>

</WebPartManifest>

Here it is done correctly

------------------------------------------------------------------------------------------------------------------------------------------------------------------------

<WebPartManifest xmlns="http://schemas.microsoft.com/WebPart/v2/Manifest">

  <Assemblies>

    <Assembly DeploymentTarget="GlobalAssemblyCache" FileName="Infusion.WebParts.dll">

      <SafeControls>

        <SafeControl Assembly="Infusion.WebParts, Version=1.0.0.0, Culture=neutral, PublicKeyToken=434425b09396c611" Namespace="Infusion.WebParts" TypeName="*" Safe="True" />

      </SafeControls>

    </Assembly>

    <Assembly DeploymentTarget="GlobalAssemblyCache" FileName="Infusion.DirectoryServicesHelper.dll">

      <SafeControls>

        <SafeControl Assembly="Infusion.DirectoryServicesHelper.dll, Version=1.0.3058.16424, Culture=neutral, PublicKeyToken=3e458cd467f1ce4f" Namespace="Infusion.DirectoryServicesHelper" TypeName="*" Safe="True" />

      </SafeControls>

    </Assembly>

 </Assemblies>

  <DwpFiles>

    <DwpFile FileName="Infusion.WebParts.DisplayUsersFromADGroup.dwp" />

  </DwpFiles>

</WebPartManifest>

Yes I know this is a pretty obvious mistake but when you get such a general error message it is touch to track down.