Adapt tax_rate_decorator to new spree version. Spree's default tax calculator now is checking line_items tax category, not the product tax category. See Spree's change here: a0a4b91f13 (diff-46e557ca8717d6ab5039470a40d00ea8)

app/models/spree/tax_rate_decorator.rb

@@ -25,9 +25,8 @@ module Spree
     # LineItems or Orders, so we mock out a line item here to fit the interface
     # that our calculator (usually DefaultTax) expects.
     def compute_tax(amount)
-      product = OpenStruct.new tax_category: tax_category
       line_item = LineItem.new quantity: 1
-      line_item.define_singleton_method(:product) { product }
+      line_item.tax_category = tax_category
       line_item.define_singleton_method(:price) { amount }
 
       # Tax on adjustments (represented by the included_tax field) is always inclusive of