Impossible activate plugin

Hi,

I uploaded the plugin fine, but when I try to activate it, I get this message displayed at the top of my dashboard :

Warning: session_start() [function.session-start]: open(/home/content/57/11937857/tmp/sess_r3ufqtmf80i0gf70svabc7njs6, O_RDWR) failed: No such file or directory (2) in /home/content/57/11937857/html/wp-content/plugins/wordpress-simple-paypal-shopping-cart/wp_shopping_cart.php on line 12

Warning: session_start() [function.session-start]: Cannot send session cache limiter – headers already sent (output started at /home/content/57/11937857/html/wp-content/plugins/wordpress-simple-paypal-shopping-cart/wp_shopping_cart.php:12) in /home/content/57/11937857/html/wp-content/plugins/wordpress-simple-paypal-shopping-cart/wp_shopping_cart.php on line 12

And none of the plugin functions work!

Could you please tell me what’s wrong? Could there be a conflict with the Responsive pro theme I use? What can I do to solve this problem?

Thanks a lot for your answer,

Ipso Imago

https://www.ads-software.com/plugins/wordpress-simple-paypal-shopping-cart/