I found 2 places that might cause this issue.
I never experienced ws before so I am not sure.
But it seems that classes/Combination.php should be modified on 2 lines:
line 383:
->join(Shop::addSqlRestriction('attribute', 'a')) should be ->join(Shop::addSqlAssociation('attribute', 'a'))
line 404:
->join(Shop::addSqlRestriction('product_attribute', 'a')) should be ->join(Shop::addSqlAssociation('product_attribute', 'a'))
Can you please test and let me know if its ok.