You cannot create new posts on a blog site that uses a customized PostCategory content-type in Windows SharePoint Services 3.0

Symptoms

Assume that you use a customized PostCategory content-type in Microsoft Windows SharePoint Services 3.0. You create a new blog site that uses the customized PostCategory content-type. In this situation, you cannot create new posts on the blog site.

Resolution

To resolve this issue, apply the following update:

2483218 Description of the Windows SharePoint Services 3.0 hotfix package (Sts-x-none.msp): February 22, 2011

Workaround

For the existing blog site, the following code example can be used to fix this issue:

using (SPSite site = new SPSite("http://localhost/blog"))  //for the Url of the blog web
{
   SPWeb web = site.OpenWeb();
   Console.WriteLine(web.Title);
   SPList postList = web.Lists["Posts"];
   SPList categoryList = web.Lists["Categories"];
   SPFieldLookup postcategoryLookup = (SPFieldLookup)postList.Fields.GetField("PostCategory");
   String lookupList = categoryList.ID.ToString("B").ToUpper();
   postcategoryLookup.LookupList = lookupList;  //this line will throw exception if the List is already set
   postcategoryLookup.LookupField = "Title";
   postcategoryLookup.Update();
   Console.WriteLine(postcategoryLookup.SchemaXml);
}

Status

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.