Looking in Apache Error Log, I am getting a lot of these errors;

[Sat Sep 3 13:03:42 2011] [apc-warning] Potential cache slam averted for key 'af79f8b8de6111780b86c7383d4ce319' in /home/itpro/www/supportforum/ips_kernel/classCacheApc.php on line 74.

We are using APC for our servers.

You should submit a ticket to ip for this and that looks like a license key so I would block some of it out if you intend to keep this post going so that nobody can see it :D

It's not a license key.... no worries.

At first glance this seems to be a server configuration issue with ACP, could be wrong, but that is where I would start.

what php and acp versions are you running?

it's a bug, set apc.write_lock = 1 and apc.slam_defense = 0 in php.ini to solve this

Without knowing all the info on your setup, it's hard to know for sure, but after looking there seems to be a bug in ACP that hasn't been fixed yet... you can add this to your config to resolve it until the bug is fixed.

apc.slam_defense = Off;

Using PHP Version 5.3.2-1ubuntu4.9
APC Version 3.1.3p1

Notice there is a newer APC-verision available. We will try to upgrade and see if that solves the issues, and if not, edit php.ini with the above tips..

Thanks!

This is why you don't use ubuntu on a server. Their focus is pushing bleeding edge software. That is not compatible with quality control. If you like use debs, aptitude and apt-get, try debian stable. It's as solid as a rock and everything has had around a year of testing before it's released.