When you get a 500 error in a webpage hosted on Debian Linux (6.0) with apache webserver and fastcgi, take a look into the apache error.log file. This can either be the global error.log or the error.log of the website where you got the error. If you find a error similar to this one:
[Fri Apr 10 15:18:05 2012] [warn] [client 192.168.0.55] mod_fcgid: HTTP request length 134926 (so far) exceeds MaxRequestLen (131072), referer: http://www.example.tld/administrator/index.php?option=com_installer
then the MaxRequestLen setting of mod_fccgid is too low. To fix that, edit the file /etc/apache2/mods-available/fcgid.conf
vi /etc/apache2/mods-available/fcgid.conf
and add or edit the line "MaxRequestLen 15728640" to set the Request Limit to 15MB. The resulting file should contain these settings:
AddHandler fcgid-script .fcgi
FcgidConnectTimeout 20
MaxRequestLen 15728640
Save the changes and restart apache:
/etc/init.d/apache2 restart