This article explains how to fix the common apache problem when updating from plesk 10 to plesk 11.
I no longer user plesk so moved this to the archive.
As my client base has grown so has my server requirements. I started using CentOS and Plesk back in April 2012 so I'm relativly new to the software. I found very quickly that I needed to disable automatic updates as that would bring down your sever at the worst of times. Manually updating Plesk allows you to quickly deal with the problems as they arise.
I assume you already ran the update and have found this site as your plesk 11 update has failed.
During the update I only received one error:
httpd: Syntax error on line 210 of /etc/httpd/conf/httpd.conf: Syntax error on line 2 of /etc/httpd/conf.d/proxy_ajp.conf: Cannot load /etc/httpd/modules/mod_proxy_ajp.so into server: /etc/httpd/modules/mod_proxy_ajp.so: undefined symbol: proxy_module
Now if you are like me and wasn't sure what proxy_ajp.conf did you will search for quite a while finding documentation.
To fix this error all you need to do is remove proxy_ajp.conf from your /etc/httpd/conf.d/ directory. Or move it to a backup location if you have more errors and might to back to plesk 10. As I understand the documentation apache no longer uses the proxy_ajp module and is no longer required in plesk 11. (but don't quote me on that).
Now just run your bootstrapper and you should be back up and running.
rm /etc/httpd/conf.d/proxy_ajp.conf cd /usr/local/psa/bootstrapper/ (cd into your version) ./bootstrapper.sh repair