If you see this error message in WSS chances are you have an active reference to custom field type but have not installed the field type. The “/_layouts/mngfield.aspx” page will throw this error in the above scenario. Two possible resolutions:
- Visit Central Administration and deploy (or re-deploy) the WSP for the custom field type. For me this is usually http://www.codeplex.com/CustomFieldControls and the Parent Child Field types.
- Remove the Site Column (SPField) objects with object model code. Requires developer skills.