Unable to customize checkout.tpl

[TKGWS]

Hello, i followed this simple guideline to customize some order forms: https://docs.whmcs.com/Order_Form_Templates#Creating_a_Custom_Order_Form_Template

I created a folder 'gws_cart' inside the templates/orderforms/ folder and created the following files:

gws_cart
├── checkout.tpl
├── configureproduct.tpl
└── theme.yaml

theme.yaml:

config:
    parent: standard_cart

I copied the configureproduct.tpl and checkout.tpl files and adapted them to my needs. While the configureproduct.tpl works fine and displays all the changes i have made, I can't get the changes in the checkout.tpl displayed 😕

diff gws_cart/checkout.tpl standard_cart/checkout.tpl 
712c712
<                         </p>HELLO FROM GWS_CART
---
>                         </p>HELLO FROM STANDARD TEMPLATE!

The checkout page always shows me "HELLO FROM STANDARD TEMPLATE".

Interestingly the configureproduct.tpl file works fine and displays the right modifications.

I set the theme in General Settings -> Ordering to the "Gws Cart" Theme, i cleared the template cache and also made sure, that no product group is configured with another theme.

Any suggestions on how to get this working or if the checkout.tpl file needs some special treatment?

Thanks in advance!