It seems a lot of servers are becoming a bit more restrictive with mod_security
and some OpenX installations either failed to initialize or you started having 403
Forbidden error pages and, or (500) blank ad spots.

I have found for some host such as HostGator it is a simple process to contact their support
and ask them to allow your/domain/openx in mod security. They may want to verify the delivery
domain / url in question. This is easily accomplished using chat, and hold time is usually not over 15
seconds. It might be noted that allowed OpenX ad impressions are subject to their TOS.

If you have a newly installed vps or a dedicated server you may have to look at your mod_security rules.
First create a secure connection to the server in question.

grep / pico /vi on the domain inside /usr/local/apache/logs error_log look for 500 / 403 Forbidden error
denoting the line number associated with file /usr/local/apache/conf/modsec2.user.conf .

You can comment out or (1) modify this line number in modsec2.user.conf and restart Apache server.

It is also possible to use SecRuleRemoveById in the virtual host config with caution.

<IfModule mod_security2.c>
<LocationMatch "/home/www/OpenX/">
        SecRuleRemoveById 900089
</LocationMatch>
</IfModule>

While users can just use SecRuleRemoveById on its own, its cautioned against. Just because a rule triggered a OpenX match does not mean that the only solution is to disable the rule entirely, consider approach 1, and editing the config line causing problems.

Either method should relieve any 403 Forbidden, and or 500 blank ad spot errors,
restart apache after making changes.

I have also found askapache has some great info concerning DreamHost and mod_security .htaccess tips that may apply in your instance.

Mike
http://www.btdsoft.com