MarketPress Upgrade Broke Checkout

After upgrade, Checkout Cart and Checkout pages are blank. cPanel error log shows:
[Fri Mar 14 13:05:47 2014] [error] [client 98.211.5.112] PHP Fatal error: Cannot use string offset as an array in /home/<domain>/public_html/wp-content/plugins/marketpress/marketpress.php on line 3498
[Fri Mar 14 13:05:03 2014] [error] [client 98.211.5.112] PHP Fatal error: Cannot use string offset as an array in /home/<domain>/public_html/wp-content/plugins/marketpress/marketpress.php on line 3498, referer: https://www.domain.com/shop/