Ok, finally I found the answer, it was in changes section of new features :

IT IS NOT POSSIBLE TO ENTER PRICES WITH TAX SINCE 5.9.6

Changes:

5.9.6: CATALooK.netStore -> Store Settings -> Price Type. If ‘tax incl.’ display is selected, prices of the products and fixed prices in product price rules must be entered now without tax!

5.9.8: CATALooK.netStore -> Store Settings -> Price Type. If ‘tax incl.’ display is selected, fixed surcharges from option selections and combinations of options and selections, fixed prices in cart price rule formulas and shipping charges from custom shipping rates must be entered now without tax!

To bulk update previously entered prices with tax included, you could execute the following script via Host -> SQL (‘Run As Script’ must be turned off)

Update {databaseOwner}CAT_Products

Set unitcost = unitcost / (1+XX/100)

(Replace XX by the tax percentage amount)

After that: CATALooK.netStore -> Store Settings -> click on the ‚Clear Cache’ link

I am not sure, but it seems to be the only good solution for all using specific tax for each country, customer, product...

So the parameter 'incl tax' is just to say if you want to display prices with or without tax, but with no impact on the way you will edit it : tax excl!!!

Hello Nicolas,
I am French too and I bought Catalook a few months ago. If you want to exchange information, that can serve us both, here is my e-mail address: etienne.bonenfant_hotmail_fr (replace _ by @ and .) .
Bon courage pour ton problème de TVA  !
Etienne.