MarketPress Checkout Error

I've got MarketPress working perfectly on one site, bet upon helping a client configure Marketpress Lite i encountered the following error upon clicking the Buy Now button.

Warning: session_start() [function.session-start]: open(/home/content/84/3570584/tmp/sess_fc8b3a2cdd45ddcd6d821892866a29d4, O_RDWR) failed: No such file or directory (2) in /home/content/84/3570584/html/mp/wp-content/plugins/marketpress/marketpress.php on line 837

I installed the full version of MarketPress and got the same error. Since the plugin works on my site, should I presume this is a plugin or theme conflict? Ufortunately this client does not want me to troubleshoot conflicts by turning off plugins. I've included theme and active plugin info below in case it may help anyone who has encountered this issue.

Any advice is greatly appreciated. Thanks!

Theme: atahualpa

Active Plugins (Don't ask, I did not create this site!): Akismet,Contact Form 7, Efficient Related Posts, Exclude Pages from Navigation, Front Page Category, HeadSpace2, Jetpack by, Map Categories to Pages, Redirection, RSS Footer, SEO Friendly Images, SEO Slugs, Share and Follow, Simple Facebook Comments For WordPress, Smart Youtube PRO, WGS Twitter Feeds, WordPress Importer, WordPress SEO, Yet Another Related Posts Plugin