Appointments+ causing fatal error in WP admin

I am trying to set his site up for a client so that she can sell her time with Appointments+ and also have a Jigoshop store. I am getting this error in the wp admin:

Fatal error: jigoshop_cart::calculate_cart_total() [jigoshop-cart.calculate-cart-total]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "jigoshop_product" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /nfs/c10/h05/mnt/149984/domains/ on line 501

This has been narrowed down to a conflict with Appointments+. When I deactivate Appointments+ (by renaming the plugin folder via ftp), the error ceases and the wp admin functions correctly. As soon as I try to reactivate Appointments+ I get the error.

This may have to do with @session_start(); in appointments.php, according to this thread that discusses the same issue with other plugins:

How can we get this to stop throwing an error? I have the latest version of all plugins and wordpress.