Ostotvsn

Great, this was the solution.

I had posponed the update for sometime, but now everything if fine.
Thank you!

Yes, downloads is a standard page.
Yes, if I create a copy of this page and it creates downloads-2 and this pages shows correctly.

Further update: possible problem found.
taxes_controller.class.php

The line 100:
/* Only subtract if coupons apply to tax. Likely in 4.0 */
/* $total_tax -= $coupon_tax; */

The problem here is that although the option to apply coupon to tax is to be introduced likely in version 4.0, already the calculation has changed.

Please revert this calculation until the option is added!

Further investigation brings that when I use the taxes_controller.class.php module of version 3.8.12, the problem is solved.

Please look into this!

In the sample at the website I used the coupon code TwentyFive, see link.