Thank you for allowing me to test your button again. I can now confirm that the notify_url value is causing the error:

<input type="hidden" name="notify_url" value="https://xxxxxxxxx/index.php?option=com_comprofiler&amp;view=pluginclass&amp;plugin=cbpaidsubscriptions&amp;cbpgacctno=1&amp;cbppdtback=e27b0e0af661ec4e69a2aa76266f1e55&amp;cbpbasket=2026&amp;do=gw&amp;result=notify&amp;cbpid=cbp5ca6052bc4bc8212274933&amp;format=raw" />
 
When I shorten this url to "https://xxxxxxxx/index.php" the button works as intended. I believe this is because, according to PayPal Standard documentation, the value can be no more than 255 characters and the url contains 283. You will have to use less characters to get the button to work.