It looks like the error is thrown every time session_start() is called. Each error file and line has that code on it.
That is not a line of code that has any setup needed. It simply starts a session, so it can't be an issue with that line of code itself.
The second two errors are caused by the first one, so the first error is the relevant one.
Warning: session_start() [function.session-start]: open(/home/content/35/2405535/tmp/sess_r586o0grmneag6sj24hn1nh484, O_RDWR) failed: No such file or directory (2) in /home/content/35/2405535/html/saraimica/WA_eCart/WA_eCart_Definition_PHP.php on line 465
So you can see the error is in the line of code: session_start()
There is definitely nothing wrong with that file itself.
The error says that there is no such file or directory, and the file or directory it is looking for is:
Now, when a session is started it is supposed to create a file unique to that session in the folder specified in the session_save_path setting from the php.ini file. Usually that is a temporary file in some sub-directory on your server. You can see that the missing file is in a directory named /tmp/
The file it creates doesn't have an extension and is named with a unique key matching the session id for the logged in use and starts with sess_.
That also perfectly matches the missing file referenced in the error. That tells me that the session file was not created properly and could not be found.
Now, am I 100% sure? No. But I'm making a very educated guess and I'd be willing to bet on it.