“Parameter Missing or Invalid” errors

[NUTRIFY]

Hello,
When adding an item to the cart, we have encountered the error Parameter Missing or Invalid: Required parameter missing or invalid. But only concerning some products such as these with variants.

Other than here described, the ID is correct
https://shopify.dev/docs/themes/troubleshooting/fix-parameter-missing-or-invalid-errors#:~:text=This....


The Theme developer says it its caused by custom codes or apps.
But we didn´t activate new one since the error occurs.

Product page with error: e.g.
https://nutrify-nutrition.com/products/flavor-drops

Can somebody help out?
Thanks in advance