I found solution for 500 errors:
We have to download this snapshot:
http://www.fastcgi.com/dist/mod_fastcgi-...952.tar.gz
and compile / install / update it. When we will do it, problem will gone!
During compiling / installing / updating a mod_fastcgi remember about apache2 install dir. Default apache2 install dir isn't proper for Debian and you have to change it in Makefile for /usr/lib/apache2 . If you will not that, you cannot run mod_fastcgi installation. How to install / update mod_fascgi you can find in the middle of the snapshot's archive.
Increasing amount of FCGI Children or other values in fastcfgi configuration doesn't improve anything in case 500 errors.
We can check that by running the apache benchmark.