When I open a particular workbook, I am getting an error message that I have
not seen befo

"Microsoft Excel cannot calculate a formula. There is a circular reference
in an open workbook, but the references that cause it cannot be listed for
you. Try editing the last formula you entered or removing it with the Undo
command (Edit menu)."

I am familiar with the usual €œcircular reference€� message, and how to deal
with it, but this isnt it. Since I am trying to open the workbook, the
message's suggestions obviously do not apply. No such message appeared
before the workbook was last saved. Without the circular reference toolbar,
or any other visual clues, it is not obvious how I can locate the offending
cell (assuming that there is one).

Am I correct in assuming that the workbook somehow became corrupted during
the last save, and that I should rescue what I can and start over with an
earlier backup copy?

Possibly useless additional information: An add-in is required to fully
recalculate; if it is present, then Excel crashes when I try to open the
file. If I unload the add-in, then I can open this file, permitting rescue
efforts. No formulas directly involving the add-in were edited in the last
session where the file opened properly.

Jerry