“Please select some product options”-error

Sitewide problem for all variable products: when a customer chooses a variable product and tries to add it to cart the popup message “Please select some product options before adding this product to your cart” occurs. In most cases it seems to be the non-default variant that gives this error. In the case of this example page the ‘200 kg’ variant.

This seems to have started in the last few days (?)

All other plugins have been updated. I deactivated the WPC variations plugin and replaced it with a few other similar plugins: the error disappears. I re-activate WPC variations and the problem is back.

I don’t want to switch to other plugins however, yours is the best.

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