Paypal Button: sandbox vs production - paypal

Question

I'm trying to integrate a Paypal Button but I'm getting error on sandbox while it looks working on production.
The code is:
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<!-- <form action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post"> -->
<input type="hidden" name="business" value="email#accountonsandbox.local">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="return" value="http://return_url">
<input type="hidden" name="cancel_return" value="http://cancel_url">
<input type="hidden" name="rm" value="1">
<input type="hidden" name="cbt" value="Back">
<input type="hidden" name="notify_url" value="http://notify_url">
<input type="hidden" name="custom" value="customdata">
<input type="hidden" name="item_name" value="item_name">
<input type="hidden" name="amount" value="6">
<input type="hidden" name="currency_code" value="EUR">
<img alt="" border="0" width="1" height="1"
src="https://www.paypalobjects.com/en_US/i/scr/pixel.gif" >
<input type="hidden" name="lc" value="it_IT">
<!-- Display the payment button. -->
<input type="image" name="submit" border="0" width="140"
src="https://www.paypalobjects.com/webstatic/en_US/btn/btn_paynow_cc_144x47.png"
alt="Paga con PayPal">
</form>
If the first line is production, it works (I see paypal's page to make payment), if I switch with line 2 (sandbox) I'm getting an internal server error from paypal's server showing the error page.
This situation is going on by more than 10 days but I can't deploy it in production without test it.
Sandbox status shows that everything is working, I don't have idea how to solve this.
UPDATE
The error is: http://imgur.com/a/vF3wC
It's in italian, translating it should be something like: There was a system error. Try later.