Thank you for your quick reply!

I found the following thread helpfull : 1.3.5 coupon tax calculation and fix .

The solution to my problem was to configure the Gift Certificate module with the following options:
- Sort order: 280 (default)
- Include shipping: false (default)
- Include Tax: true (default)
- Re-calculate Tax: None
- Tax Class: TVA

Not a bug. Just a training matter...

Olivier