Error 404 , /midgard not found.

This is a good moment to set Apache log level to debug. Do not try to find /spider-admin , /midgard or /aegir dir on your filesystem under DocumentRoot set in Virtual host configuration file. Instead of this , open main Apache configuration file and set log level to debug. It is set by simple directive:

LogLevel  debug

Stop Apache and start again. Request midgard page and take a look at log file.
In many cases you may find debug errors like this one:

[debug] midgard-apache2.c(1314): [client xxx.xxx.xxx.xxx] Midgard: regenerating /var/local/cache/midgard/7-57-12-0.php: cannot open, assuming non-existant
[debug] midgard-apache2.c(2253): [client xxx.xxx.xxx.xxx] Midgard: Couldn't create cache file

It simply means that one of the directory set by MidgardPageCacheDir has not even executable bit.
You should check every dir in this path. The most probably is that the 'cache' directory itself has drwx------ permissions ( the most probably for system when midgard is self compiled and with prefix other than /usr ).
Change permission to drwx--x--x using chmod:

chmod 711 /var/local/cache

Note , that we use only x bit instead of rx as we do not want Apache user to be able to list this directory.

Entry added also in troubleshooting section on www.midgard-project.org