The same issue occurs to me in my page with some variable products (not all).
For example, I have one product (between others) that I couldn’t add to my cart because it wasn’t possible to select the ‘colour’ attribute, and also some sizes, but there was stock.
I rolled back the version of the pluggin from 1.0.11 to 1.0.10 and the error disappears.
Here is the product page around I talked about: https://www.ropamotobmw.com/guantes-gs-rallye/
Thanks a lot.