There is no access log for FastCGI, because it isn't a program, it's a protocol. For debugging the PHP fastcgi handler, I've usually resorted to
strace -- it usually shows me what file is trying to be accessed, and it's not hard to work out how it went wrong from there. Nginx's request processing debug logging is often instructive, too.
Using strace for this is pretty straightforward -- you just strace the PHP FCGI workers and limit yourself to read/write calls with
-e trace=read,write. Upping the string print size with
-s 4096 is also a good idea, so you get the entire FCGI packet rather than just the first few bytes.