Workbook contains links that cannot be updated

When I opened a workbook that I hadn't used in awhile, I got a pop-up error message saying, "This workbook contains one or more links that cannot be updated". I've attached a screen shot of that pop-up and the ones that followed.

I clicked on the Edit Links button and was shown the Edit Links dialog. It listed one entry, my add-in module, with an Update setting of "A" (automatic?) and a Status setting of "Unknown" (what does that mean?). Pretty cryptic so far.

Someone in another thread said to open the Name Manager, so I did. I found 6 entries with "#REF" in the Value column. In the Refers To column, they all had a path to a workbook that doesn't exist. I opened the sheet in the Scope column and searched for the Names that had a value of "#REF". None of them were referenced on that sheet or any sheet in that workbook.

My main question is what to do with them? It seems like I can just delete them, since they don't exist and aren't referenced.

My second question is how did this happen?

Thanks

OK, thanks