the file cannot be saved because some properties are missing or invalid. use the document inforamation panel to provide the correct property values. errors for required properties are marked with a red asterisk

Hi Experts

I am receiving an error when trying to save a file from Word into SharePoint: "This file cannot be saved because some properties are missing or invalid"

I have many Content Types for this Document Library: all documents have got exactly the same required fields, all required fields are displayed in the Document Information Panel.

but I cannot figure out where that would come from? If I go to manage the Content Types and check the list of fields, I can clearly see which fields are required and which aren't.

Is there something I am missing? Or could one Content Type be corrupt? If so, how do I fix it without messing up all the files that have already been posted using this Content Type?

Please advise asap,

Kusum