Lost Menu Events after opening a Modal Form. WHY??? Please help!!

[Peter]

I created a COM Add-In in C# for Excel XP, which installs
an additional popup menu and toolbar. Now I receive the
menu and toolbar click events without problem and I can
handle the clicks in my code. But if I open an empty modal
form in my click event with the following code

AboutForm dlgAbout = new AboutForm();
dlgAbout.ShowDialog();

the form opens correctly. After closing the form, the
program returns to my code without any error. But after
that I can click where I want, I never receive any
customer menu or toolbar event again! All other Excel
menus and also Excel seem still to work fine.

* What is wrong with my code??
* And why I never receive an event after opening a dialog
window?

My software:
VS 2003
Office XP SP 2

Any help is greatly appreciated. Many thanks in advance!

Peter