MarketPress Fatal Error for PayPal Express Checkout

I got a Fatal Error when unchecking PayPal Express Checkout. "Fatal error: Cannot use string offset as an array in /home/chester/public_html/wp-content/mu-plugins/marketpress.php on line 342"
I have repeated this error on two different subdomains just by checking then un checking the PayPal Express Checkout Option and then hitting save. My only Solution is to delete the site and recreate it. How would I fix this issue otherwise? Would I have to edit the database or something. Also is there a way to backup the individual sites on a multisite install if something like this happens and I don't want to remake the site?
Thanks