Plugin keeps showing error after theme update

DutchPC

Hello,

I use the plugin RnB - WooCommerce Booking & Rental. https://redq.io/rnb

This worked just fine, but after the last theme update i get an error message.

Strange thing is that the plugin still works.

I did some trouble shooting:

1 - disabled all plugins, did not work

2 - updated all plugins and theme, did not work

3 - change the theme to storefront and the message was gone.

So that made me think the issue is within the BeTheme. The error show on top of the page after a reservation. But the product is added to the cart and you can go to checkout. But visitors think because of the error message that the product is not availeble.

The message says: Sorry, this quantity is not available

https://hulshorstverhuur.nl/winkel

What can i do?

Thanks!