Subscribe2 is awesome, I've been using it for several years now without issue.
I just upgraded to WP 3.5 and went ahead and upgraded subscribe2 while I was at it.
I'm actually not entirely sure when subscribe2 stopped working, although I was getting subscriptions as recently as 2 weeks ago. But, just in case the upgrade is a red herring, I thought I'd throw that out.
Basically, the subscribe widget and the post page quite working. I get:
Sorry, there seems to be an error on the server. Please try again later.
I can send test emails just fine.
I changed the shortcode, as I was using the old version previously, but that didn't help either.
Any ideas on where to look?