I’m also seeing this issue, someome on our admin updated today to 2.1 and im looking at the error right now.. here is what I have in the error logs.
[Sat Feb 22 18:23:31 2014] [error] [client 72.47.20.114] WordPress database error Duplicate entry '4' for key 'index_in_wallet' for query INSERT INTO
e6iqlf_bwwc_btc_addresses\r\n (
btc_address,
origin_id,
index_in_wallet,
total_received_funds,
received_funds_checked_at,
status) VALUES\r\n ('sdfgW6sdfgsfdgfFUDuahXGsdfgsdyEFw', 'electrum.mpk.sdfdfgdfgdc4sdfgsdg28732bf11cddf9sdfgsdfg9cb', '4', '0', '1393093411', 'unused'); /* From [example.com/checkout/] in [/nas/wp/www/cluster-2092/example/wp-content/plugins/bitcoin-payments-for-woocommerce/bwwc-utils.php:336] */ made by require('wp-blog-header.php'), require_once('wp-load.php'), require_once('wp-config.php'), require_once('wp-settings.php'), do_action('init'), call_user_func_array, WC_Form_Handler->checkout_action, WC_Checkout->process_checkout, BWWC_Bitcoin->process_payment, BWWC__get_bitcoin_address_for_payment__electrum, BWWC__generate_new_bitcoin_address_for_electrum_wallet, referer: https://example.com/checkout/
the error is on this line..
$query =
"INSERT INTO <code>$btc_addresses_table_name</code>
(<code>btc_address</code>, <code>origin_id</code>, <code>index_in_wallet</code>, <code>total_received_funds</code>, <code>received_funds_checked_at</code>, <code>status</code>) VALUES
('$new_btc_address', '$origin_id', '$next_key_index', '$funds_received', '$received_funds_checked_at_time', '$status');";
$ret_code = $wpdb->query ($query);