One of your plugins is using PHP Sessions and the error is because PHP can’t read or write to the directory set in your php.ini as the session.save_path.
I don’t think it is related to the pricing tables plugin because I looked through the code and it doesn’t use sessions.
Please disable or delete all plugins and switch to the default theme (TwentyEleven since 3.3). After making these changes does the problem still occur? If no, re-enable everything one by one to see which component causes the problem.