This issue usually happens when you are using "encrypted" item button code. When you copy and paste the code from the PayPal button creator, your HTML Editor may add a space or not paste the code exactly as it was generated. Suggest copying the code again but, this time paste the code to a blank web page, save the page locally with a name and an extesion of .htm or .html. Then open the file with your web browser and test it. It should work OK. Then compare the working code to what you have on your live web page and see if find the difference.
Below is an example of a blank web page. Last, whenever you are testing item button code, always delete your browser's cookies, temp files and history before testing the code. This way you get a clean start after making any code changes. Should you not do this each time, the data retained from the previous cookie may cause unexpected results.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Test</title>
<!-- START META TAG SECTION -->
<meta charset="utf-8">
<meta http-equiv="Content-Type" content="text/html;charset=utf-8">
<meta http-equiv="Content-Language" content="en">
<!-- END META TAG SECTION -->
</head>
<body>
<!-- INSERT TEST CODE HERE -->
<!-- END TEST CODE SECTION -->
</body>
</html>
