Hi,
I fixed the problem, at the moment it works for me. I don't know if this is the best solution but you could try it out.
change the next file at row 41:
\themes\classic\templates\catalog\_partials\product-add-to-cart.tpl
I just removed "disabled out of the line so it wil be permanently active.
original line(row 41):
<button class="btn btn-primary add-to-cart" data-button-action="add-to-cart" type="submit" {if !$product.add_to_cart_url || $product.quantity_wanted>$product.quantity}disabled{/if}>
my version:
<button class="btn btn-primary add-to-cart" data-button-action="add-to-cart" type="submit" {if !$product.add_to_cart_url || $product.quantity_wanted>$product.quantity}{/if}>
Maybe someone else has an better solution.