Problems with VAT calculation at production system

Hello,
I have a very important, productive problem. I ask for processing with the highest priority. An incorrect VAT rate is currently being displayed and calculated in my productive online shop.

The following situation can be simulated on my test environment. The test environment is a 100% image of the productive system.
The test environment can be accessed at the following URL:
https://test.photovoltaik4u.com

I have not made any changes in the central settings for VAT – here 19% VAT is set for Germany (see screenshot VAT-problem00.jpg)

To simplify the presentation, I have created a test product with a price of 100 euros (https://test.photovoltaik4u.com/produkt/test) – see also screenshot VAT-problem01.jpg)
In the displayed text of the product, the VAT is correctly displayed as 19%.
Now, when I add the product to the shopping cart, the correct VAT rate is shown as 19%, but the result is incorrect when the VAT is calculated. It is calculated with about 16% (see screenshots VAT-problem02.jpg and VAT-problem03.jpg).
The problem also exists in the checkout process (see screenshot VAT-problem04.jpg).

What did I do before noticing VAT being calculated incorrectly:
I wanted to activate the “Paypal” payment method.
1. In order to activate PayPal, I deactivated the productively used theme (R-Energy Child) in the test environment and activated the standard theme (Shopper).
2. Now I have set up and activated the “Paypal” payment method.
3. With the Theme Shoper and activated Paypal I made 2 test purchases and checked whether the payment works and the invoice is created correctly. (Unfortunately, I didn’t look directly at the VAT, only whether the payment method had been changed correctly. Afterwards, I looked at the generated invoice PDF. The problem with the VAT already existed here).
4. After everything was fine from my point of view, I edited my productive online shop and activated Paypal.
5. Now I made another test purchase in the productive system and noticed the problem with the wrong VAT.

I now have the wrong VAT calculation in the productive system and urgently need help.
I’ve already checked all the settings and unfortunately I can’t find the error.

Thank you very much for your help.

Greetings,
Jens

The page I need help with: [log in to see the link]