File Links aren't working like before

I'm very familiar with linking files to pull info from one spreadsheet into another, been doing it for years. I'm having a new problem. I have a target spreadsheet which references a closed spreadsheet. Usually a target sheet simply shows the last saved value from the source sheet. Now, the target sheet displays either a #VALUE! or #REF! error. When I open the source sheet to investigate, there's nothing wrong with the referenced cell - it's there, it's calculating correctly, no problems. And in the target sheet, the error messages immediately go away. But tomorrow when I come back to work on it, again the target sheet will show those error messages until I re-open the source sheet. This is totally new behavior for my experience.

Also, in my target sheet, there are 2 or 3 other source sheets linked in. None of these sheets give me an error, even if they're closed when I'm working on the target.

Both target & source sheets are set for automatic recalc, and iterations are turned off.

Any help is appreciated!

Thanks for your reply, but I don't understand what you mean. In my first paragraph, I said "When I open the source sheet to investigate, there's nothing wrong with the referenced cell". So, yes, I think I checked the connections. What else should I have done?

There is nothing under "Data\Queries & Connections". Under "Data\Workbook links" there are a total of 4 other spreadsheets. None of them give me a #VALUE! error when they're closed. Note that, as soon as I open the problematic spreadsheet, the VALUE error goes away, so I think that proves the link is valid.

SUMIF.

All the other source references are a simple cell reference (=Filename\cell reference) or adding cells from the source workbook (=sum(filename\cell reference + filename\cell reference). I'll bet that Excel can't use the SUMIF function in workbook A when it references criteria that are in workbook B.

I think you solved it! What do you think?