Sajax: lost parameters passed to php function

Hi,
i write an entire site using sajax and php and I found it very powerful
and useful.
But yesterday my client had formatted the ubuntu server and when server
is come back online a strange thing happened.
Sajax seems to work fine (i use the debug mode) while send parameters
to the exported function but when the server function has been called
the parameters sended by javascript are lose and the function receive
the string "Array".

I miss some php/apache module?
Maybe some security module installed don't let pass the parameter?

Any other idea are appreciated...

Thanks

MicKy

This is my code:

<?php
require("include/Sajax.php");
function checkuser($usr) {
//The paramenter $usr receive always the string "Array"
$query="SELECT * FROM users WHERE username = '".$usr."'";
query($query,$res,$rows);

if ($rows > 0) {
// TODO: ...
}
}

sajax_init();
//$sajax_debug_mode = 1;
sajax_export("checkuser");
sajax_handle_client_request();
?>

<?php sajax_show_javascript(); ?>

function do_check_ris(ris) {
document.getElementById("ris").value = ris;
}

function do_check() {
// get the folder name
var usr;
usr = document.getElementById("txtNick").value;

x_checkuser(usr, do_check_ris);
}
</script>

// form...
<input name="txtNick" type="text" id="txtNick" /> <input type="button"
name="check" value="check!" onclick="do_check(); return false;" >
<input type="text" name="ris" id="ris" value="" >