Most likely there was an update made to the version of PHP on the server that introduced a PHP bug whith session management where session information is lost when the header() function is used to redirect to the cart page.
On any page with an add to cat button, look for the following code block:
<?php
// WA eCart Redirect
if ($[Cart Name]->redirStr != "") {
header("Location: ".$[Cart Name]->redirStr);
}
?>
where [Cart Name] is the name of the ecart object.
and add session_commit(); before the header function:
<?php
// WA eCart Redirect
if ($[Cart Name]->redirStr != "") {
session_commit();
header("Location: ".$[Cart Name]->redirStr);
}
?>