You know this is funny because I just installed s2 and it did the same thing to my site.
It was fine before but now 403 only for this one thing.
Seems very strange that I get the same issue.
I also have another site with no s2 and it is working fine.
So I kinda agree with adput that s2 did something.
May not happen every time but it does happen.
I am running buddypress 1.5.5 on wp 3.2.2
needless to say I will not install on another site without using in a test site first.
The worse thing is I got no response to any questions from s2.
Matter of fact I did not see a single answer to any of the hundreds of questions being asked.
Seems to me customer service is number one.
At least that is the way I built my company.
while you can't solve all things. At least reply to the thing and give some response. don't just ignore it all together.
Needless to say i won't be buying Pro anytime soon.