Q:

Zend / Apache2: Getting 302 Found when requesting url several times

I am programming a REST API with Zend framework. 
When calling the url several times (e.g. 1000 times with 1 request per second), in about 0.2 % of the cases instead of getting 200 OK as a response I get 302 Found - so a redirect to a different page. 
Here is the entire server response:
302 Found Date: Mon, 04 Mar 2013 11:56:04 GMT
Server: Apache/2.2.17 (Ubuntu)
X-Powered-By: PHP/5.3.5-1ubuntu7.11
Set-Cookie: PHPSESSID=ui9r8jqa63dbom8osknso6eea5; path=/
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Location: /de/default/index/index/error/500
Vary: Accept-Encoding
Content-Length: 0
Content-Type: text/html; charset=utf-8

So Zend redirects to the error 500 (internal server error) page.  The question is why - and I just can't figure it out...
The php page that is called inserts one row into a MySQL database and returns a JSON string - that's all. 
Apache2 has about 20 concurrent connections open and the server load is <<1 so I really do not understand why the requests cause problems. 
I know this is a really difficult problem to remote diagnose, but good guesses and recommendations how to solve this are more than welcome!  Thanks. 
This is the apache vhost config as requested by @chris:
<IfModule mod_ssl.c>
<VirtualHost mydomain.tld:443>
    ServerAdmin webmaster@localhost

    DocumentRoot /var/www
    ServerName www.mydomain.tld
    ServerAlias mydomain.tld *.mydomain.tld
    <Directory />
        Options FollowSymLinks
        AllowOverride All
    </Directory>
    <Directory /var/www/>
        Options Indexes FollowSymLinks MultiViews
        AllowOverride All
        Order allow,deny
        allow from all
    </Directory>

    ScriptAlias /cgi-bin/ /usr/lib/cgi-bin/
    <Directory "/usr/lib/cgi-bin">
        AllowOverride None
        Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
        Order allow,deny
        Allow from all
    </Directory>

    ErrorLog /var/log/apache2/error.log

    # Possible values include: debug, info, notice, warn, error, crit,
    # alert, emerg. 
    LogLevel warn

    CustomLog /var/log/apache2/ssl_access.log combined

#   RewriteLog "/var/log/htaccess.log"
#   RewriteLogLevel 5

    Alias /doc/ "/usr/share/doc/"
    <Directory "/usr/share/doc/">
        Options Indexes MultiViews FollowSymLinks
        AllowOverride None
        Order deny,allow
        Deny from all
        Allow from 127.0.0.0/255.0.0.0 ::1/128
    </Directory>

    #   SSL Engine Switch:
    #   Enable/Disable SSL for this virtual host. 
    SSLEngine on
    SSLOptions +StrictRequire
    SSLCertificateFile /etc/apache2/ssl/cert_2013_2014.crt
    SSLCertificateKeyFile /etc/apache2/ssl/www.mydomain.tld.key
    SSLCACertificateFile /etc/apache2/ssl/intermediate.crt

</VirtualHost>
</IfModule>

A:

Wow - that was a totally unexpected problem and really hard to figure out...
@AdrianWorld helped me get on the right track.  In the Zend ErrorController I output the error message and found the following exception:
ps_files_cleanup_dir: opendir(/var/lib/php5) failed: Permission denied (13) Array

That apparently is a pretty common problem as described and solved here. 
The variable session.gc_probability was set to 1 in the php.ini which means there is a 1% probability for the garbage collector to run and clean up the directory /var/lib/php5 where the php sessions are stored.  Apparently this folder is not writable by www-data resulting in the mentioned error and throwing the Zend exception. 
Since session.gc_probability sets only a probability the error occured randomly making debugging pretty difficult. 
Anyways, I'm happy it is solved - thanks for all the hints and guesses :)

