Sorry, some further information:

a) If I have a product price of 8280 and no shipping costs, the tax is displayed correctly as 753.
b) If I have a product price of 8280 and shipping costs of 910 (total of 9190), the tax is displayed as 836, it should be 835.
c) If I change the product price to 9190 (same like total in above) and free shipping, the tax is displayed correctly 835.

So it seems there is something wrong with the tax calculation of the shipping costs.