Here is the fix:

Find line 131 in /includes/functions/functions_prices.php which looks like this:

Code:
// is there a products_price to add to attributes
      if($customers_group) {
        if($customers_group == "Group A" && $product_check->fields['products_group_a_price'] != 0) {
          $products_price = $product_check->fields['products_group_a_price'];
        } elseif($customers_group == "Group B" && $product_check->fields['products_group_b_price'] != 0) {
          $products_price = $product_check->fields['products_group_b_price'];
        } elseif($customers_group == "Group C" && $product_check->fields['zen_products_group_c_price'] != 0) {
          $products_price = $product_check->fields['products_group_c_price'];
        } elseif($customers_group == "Group D" && $product_check->fields['zen_products_group_d_price'] != 0) {
          $products_price = $product_check->fields['products_group_d_price'];
As you can see the sql select for group c and d have 'zen_' already in the code thus the select statement is zen_zen_products_group_d_price when it should be zen_products_group_d_price assuming your table prefix is zen_ .. the fix is:

Code:
// is there a products_price to add to attributes
      if($customers_group) {
        if($customers_group == "Group A" && $product_check->fields['products_group_a_price'] != 0) {
          $products_price = $product_check->fields['products_group_a_price'];
        } elseif($customers_group == "Group B" && $product_check->fields['products_group_b_price'] != 0) {
          $products_price = $product_check->fields['products_group_b_price'];
        } elseif($customers_group == "Group C" && $product_check->fields['products_group_c_price'] != 0) {
          $products_price = $product_check->fields['products_group_c_price'];
        } elseif($customers_group == "Group D" && $product_check->fields['products_group_d_price'] != 0) {
          $products_price = $product_check->fields['products_group_d_price'];