<p>I just wrote a new web part and now I am getting this error when I try to deploy them on my non-dev servers:</p>

<blockquote>
  <p>the default namespace
  '<a href="http://schemas.microsoft.com/WebPart/v2" rel="nofollow noreferrer">http://schemas.microsoft.com/WebPart/v2</a>'
  is a reserved namespace for base Web
  Part propertiees. Custom Web Part
  properties require a unique namespace
  (specified through an
  XmlElementAttribute on the property ,
  or an XmlRootAttribute on the class).</p>
</blockquote>

<p>I am writing the web parts into CAB files and deploying them with this:</p>

<pre><code>stsadm -o addwppack -filename web_part_name.CAB -url http://your_url_here -globalinstall -force 
</code></pre>

<p>Everything works fine until I try to add the web part, then I get this error in a popup.  It works just fine on my dev VM...?</p>

<p>Any ideas would be appreciate, thank you.</p>

A bit of a educated guess here but anyway :-
First check that you have XmlRoot attribute like this in your web part
```
```
[XmlRoot(Namespace = "Your.Namespace")]
public class YourWebPart: WebPart
{
...
```
```
and XmlElement attribute on your custom properties
```
```
[DefaultValue(0)]
    [WebPartStorage(Storage.Shared)]
    [Browsable(false)]
    [XmlElement(ElementName = "YourProperty")]
    public Int64 YourProperty
    {         
       ...
    }
```
```
This error is happening when .NET is attempting to desterilize the data from the .DWP file and set the custom properties in your web part.
I suspect that the error may have nothing to do with namespace conflicts as SharePoint sometimes tends to fall back to an error messages that can be a red herring.
I would firstly examine your .dwp file. Do you have any custom properties set there, if so remove them and retest.
Comment out the custom properties in your web part code one by one and retest at each step.
I think you will find one of these is causing the problem - exactly why is the next question!