Excel recalc triggered on workbook_open

[Conceptor]

Hi,

I created a XLA addin that is already activated (and
loaded) when Excel XLS workbooks are opened. These XLS
workbooks have cell formulas that refer to a macro called
Macro1 I put in the XLA code. Problem: When a XLS
workbook opens, Excel automatically recalculates the
workbook cell formulas that use Macro1. I need to cancel
this recalculation because when loading, Workbooks don't
have all the information needed to evaluate the Macro1
return value.

I tried to cancel the calculation by using:
application.calculation = xlCalculationManual
in the Workbook_Open events (catched from the XLA). When
loading the XLS workbook, Excel crashes with the following
error message:
Execution error 1004: the 'calculation' method of
the 'application' object failed.
I never was able to find out where this automatic
calculation came from. No cell has been modified on load,
none of the conditions that would normally trigger a
calculation are present on Workbook_Open.
So, what happens? How do I deactivate the auto
calculation on workbook load, and more importantly how do
I reset the auto calculation after the workbook_open event?

Thanks for any tips

C.