Encrypted paypal button stopped working

Just today I got a notice from a potential new subscriber letting me know that the PayPal button leads to an error:

The seller accepts encrypted website payments only. You cannot pay the seller through un-encrypted buttons. Please contact your seller for more details.

I have not changed any options in PayPal or in the options pages of the s2member plugin that required the encrypted button setup a year ago when I installed it. The last successful user registration with payment using the button was 31 January 2013 (we don’t exactly have a very high registration volume).

I have not installed any new plugins since then, so it shouldn’t be interference from a plugin. I checked that the s2member >> PayPal Options >> Enable Button Encryption? button is still ticked Yes. The associated API username, password, and all other information is present and accounted for. I had not generated a new button code recently – it was the same shortcode as before.

What shifted beneath the surface to make this no longer work? I am completely baffled.

Here’s the shortcode for my button, if it’s relevant:

[s2Member-PayPal-Button level="1" ccaps="" desc="Recurring subscription / $10.00" ps="paypal" lc="" cc="USD" dg="0" ns="1" custom="www.nasarracenia.org" ta="10.00" tp="1" tt="Y" ra="10.00" rp="1" rt="Y" rr="1" rrt="" rra="1" image="default" output="button" /]

Kind regards,
Ryan

In looking around at other posts on this topic, I followed their advice and tried:
1) changing themes to the twenty-eleven theme.
2) Disabling all but the s2member plugin.

Neither of these worked, so I don’t see how this could be the result of interference with the shortcode parsing. And as I said in the original post, I have not changed anything or updated the theme or other plugins recently.

I checked the API log (paypal-api.log). Bots seem to hit the button all the time (?) though they never complete a transaction, so the last “success” in the log was Thu Mar 14, 2013 8:40:05 am UTC:

LOG ENTRY: Thu Mar 14th, 2013 @ precisely 8:40 am UTC
PHP v5.2.17 :: WordPress® v3.5.1 :: s2Member® v130221
Memory 38.44 MB :: Real Memory 38.75 MB :: Peak Memory 38.55 MB :: Real Peak Memory 38.75 MB
www.nasarracenia.org/join/
User-Agent: Mozilla/5.0 (compatible; Baiduspider/2.0; +http://www.baidu.com/search/spider.html)

The first error was Thu Mar 14, 2013 6:19:59 pm UTC:

-------- Output string/vars: ( Thu Mar 14, 2013 6:19:59 pm UTC ) --------

array (
  '__error' => 'Error. Please contact Support for assistance.',
)

There is nothing different between these two attempts not even a day apart. All of the button parameters remained the same and the website did not change, nor did our paypal settings. Hence my complete and utter confusion.

Anyone have a guess or another possible solution I could try?