Thanks Ismael, I have done the temporary fix and it has worked. The stuff in the thread about making it a permanent fix by using the child theme was confusing and unclear whether it would work so I have not followed it. If there is a clear instructions on how to make this a permanent fix please post here.
Thanks
Rob

I have installed a testing site. First I updated Woocommerce to 2.6.2 and the problem was still there. I then changed the theme to Twenty Sixteen and the problem disappears. I would say this indicates that there is a conflict with Enfold theme with Woocommerce and Woocommerce Measurement Price Calculator.
Testing site: http://kindertimberproducts.co.uk/staging/
Feel free to log in and investigate. Please use the same login previously supplied for staging site.
I replied to Woocommerce letting them know my findings This is their response:

…With regard to the custom theme you are using, please have the developers update their theme to work with the latest version of WooCommerce. Often times this is not the case resulting in issues like this. You could also consider an industry standard WooCommerce theme like StoreFront which is free of cost for your store.

Thanks
Rob

This is the reply I got from Woocommerce. Disabling the theme and other plugins that they suggest is not an option on a live shop. How do you think I should reply?
Thanks
Rob

Andrew Wikel (WooThemes)
Jul 14, 09:40 EDT

Hey there Rob,

Typically issues like this may be caused by either a conflict with your theme or with another plugin. How we usually address this type of issue is to have you temporarily switch to a theme we know works, such as Storefront and disable all plugins except for WooCommerce and the plugin in question . If that resolves the issue, then slowly re-enable features until you find the one that’s causing the conflict.

If that doesn’t help can I ask you to:
1. Confirm that you have performed the steps above
2. Grant us permission to perform the same troubleshooting steps as outlined above

Cheers!
Andrew W – WooCommerce Ninja
WooThemes Support

Hi
I have tried disabeling WC Fields Factory and WooThemes Helper plugins but it had no effect. I cannot disable the others as they are fundamental to the running of the shop.

Please can you confirm somebody is looking into this. My client is losing sales and getting very stressed.

Thanks
Rob

No Luck I’m afraid. I have followed the instructions on https://kriesi.at/support/topic/woocommerce-2-6-problem-urgent/#post-650161, cleared all cache with wp super cache and the problem persists. i.e. I get “Please match the requested format Qty” and it doesn’t add to basket.

Please help, the shop is non-functional on the most popular browsers!

I am having the same problem after updating Woocommerce just now on http://kindertimberproducts.co.uk.
Some points:
– I am also using Enfold
– I have these Woocommerce plugins:
WC Fields Factory
WooCommerce Measurement Price Calculator
WooCommerce WorldPay Gateway
WooThemes Helper
– It works okay on Safari on Mac and PC and i.e. on PC
– I have tried it 2 PCs on Firefox and Chrome and I get the pop-up on both.
Thanks
Rob

Are you sure there is no way to have the featured products in the product slider. It seems crazy. I can’t see the point of a product slider that doesn’t allow you to select which products it shows. I use other WP themes and in the product slider there is a drop down that allows you to select featured, best sellers etc. I have tried what you said above and I agree with Runek, it is no workaround at all.
Thanks
Rob