What does "write failed: No space left on device" mean?

In a shared hosting, change the location where sessions are saved.

Create the directory: /public_htm/tmp, give the permissions (777) <- then you have to tune it (for security).

Create a file .htaccess in the directory /public_htm/tmp with:

order deny, allow
deny from all
allow from 127.0.0.1

And in your PHP script, on top (in your front controller):

session_save_path(dirname($_SERVER['DOCUMENT_ROOT']).'/public_html/tmp');

Important: You must be added before session_start()

Check this with a script:

<?php
session_save_path(dirname($_SERVER['DOCUMENT_ROOT']).'/public_html/tmp');
session_start();
$_SESSION['Text'] = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed quis nunc eu ut elit eget adipiscing facilisis turpis."
phpinfo();
?>

Check if changed the directive "session.save_path"

I'm assuming you're hosting your site on a shared hosting provider Basicly the error means that the server has run out of disk space, so it can't complete the sql commands

Just call/email the hosting company and they should fix it for you

df -h

to look /tmp space left.

If /tmp is over 100%

cd /tmp

rm -r ./* -f

And you can save sessions again.