Invalid URL Parameter error in WSS 3.0

You get this error when you use custom spcf page and custom layout pages for web parts. In my case, the problem was with custom web part spcf page. The full error read:

http://websiteURLhere:8888/_vti_bin/owssvr.dll?CS=65001

Invalid URL Parameter

The URL provided contains an invalid Command or Value. Please check the URL again.
Troubleshoot issues with Windows SharePoint Services. 

Follow this tutorial to the point, it helped me to identify where I went wrong (it was in the value field of the optio n element of the template list seen on the screen by the way!)

Creating Custom Web Part Page Templates for Microsoft SharePoint Products and Technologies

http://msdn.microsoft.com/en-us/library/dd583147%28office.11%29.aspx