What causes Excel to raise an error about "one or more formula references" when saving?

The problem was caused by a chart with empty data ranges. This potential cause was mentioned in this forum.

This is caused by empty values in named ranges/cells.

To debug, go to Formulas > Name Manager. Look for Named Ranges with an empty value. Either change the where the reference points (the "Refers to:" column) such that it pulls in a value, or delete the reference if it is not needed.

Be careful deleting named ranges, as you can cause some downstream #REF errors.