MarketPress: slow checkout and PayPal issues

I’m experiencing the following issues with MarketPress plugin:
1. The checkout process is increadibly slow. I have contacted my host to increase php memory, but this didn’t help in a slightest.
2. PayPal Express checkout does not work. This is the message I’m getting: “There was a problem connecting to PayPal to setup your purchase. Please try again”. PayPal claims that everything should work on their end. Host claims that there are no restriction/banes on their end to terminate session.
This is the info I get when debugging:
“Trying to get property of non-object in /hermes/bosoraweb155/b746/ipg.sophysamcom/wp-includes/general-template.php on line 1644 Notice: Trying to get property of non-object in /hermes/bosoraweb155/b746/ipg.sophysamcom/wp-includes/post-template.php on line 1272
Notice: wp_enqueue_script was called incorrectly. Scripts and styles should not be registered or enqueued until the wp_enqueue_scripts, admin_enqueue_scripts, or init hooks.”

I have contacted WPMU before placing this request (live chat and support), but got only a round about answer that it might be my host. Now, since this is cleared up, I would like to get to the bottom of these issues and make the plugin work.