I had a .NET Error when creating a custom field type in my SharePoint list:
Microsoft.SharePoint.SPException: Field type FolderField is not installed properly. Go to the list settings page to delete this field.
at Microsoft.SharePoint.SPFieldCollection.GetDotNetTypeForFieldType(String typeName)
at Microsoft.SharePoint.SPFieldCollection.CreateNewFieldInternal(String typeName, String displayName)
at Microsoft.SharePoint.ApplicationPages.FieldNewPage.Save()
Reason therefore was an error in my fldtypes_MyCustomField.xml.
There I had a wrong FieldTypeClass. It should be this way:
<Field Name="FieldTypeClass">[Class with Namespace Test.MyCustomField], [Assembly-name], Version=1.0.0.0, Culture=neutral, PublicKeyToken=b426d5u8827cb6b6</Field>
