This is the shopper group table content I have currently:
INSERT INTO `jos_vm_shopper_group` (`shopper_group_id`, `vendor_id`, `shopper_group_name`, `shopper_group_desc`, `shopper_group_discount`, `show_price_including_tax`, `default`) VALUES
(5, 1, '-default-', 'This is the default shopper group.', 0.00, 1, 1),
(6, 1, 'Gold Level', 'Gold Level Shoppers.', 0.00, 1, 0),
(7, 1, 'Wholesale', 'Shoppers that can buy at wholesale.', 0.00, 0, 0),
(8, 0, '-default-', 'Our shop', 0.00, 1, 1);
Two times default as shopper_groups_name should not really be the issue (even though default can only be chosen as group once), but one shopper having vendor ID 0 connected to it might be an issue. And then there is the strange business of the usage of two shopper groups IDs for storage and display of prices when the price is added to the database for the first time. I think that there are two default groups and in the second half of the process query looks for shopper group with vendor ID 1 and one that is default. Then you get shopper group 5. The fact that shopper group 5 and 8 are both default (not just as a group name) is not good..