How many EPiServer instances are you running? Each IIS instance requires its own site license in CMS 6.

Also are you only getting the error on one machine? 

If you are referring to sites within EPiServer.config, 2.  The error only occurs on the 'slave' server where we have one license used by both sites as advised by EPiServer.

I would contact EPiServer and ensure that the license you have is licensed for 2 sites. This is an exception for CMS 6 as normally each license is for an individual site, even with load balancing.

Hi Daryl, 

Could you please verify what limit do you have for <MultiSiteLimit> in your SLAVE license.config. As per error message, it seems like you are using single-site license on multiple sites. 

Did you remove a existing site which was using the same license ? 
http://world.episerver.com/FAQ/Items/License-violation-after-removing-a-second-site/

The license file we have from EPiServer does not contain a MultiSiteLimit tag.  Can we assume this limit is the default 15?  

Nope, default is "single-site" license. Now you need a new license.config for your 2nd site or ask your sales contact at EPiServer to resend you a license file which supports two sites <MultiSiteLimit>=2  (if that was the deal).

15 sites per licence was the model for CMS 5 enterprise.

From my experience CMS 6 is designed to work with individual licences, one per site. EPiServer can issue a multi-site site licence but this is an exception rather than the rule.

Shahid can you confirm?

Seems you beat me to it Shahid!

Many thanks for your replies.  Based upon your comments, we are contacting EPiServer to understand their previous correspondence we have had with them on our licensing agreement.

Thanks again. 

To conclude, your replies are correct.  A license is needed per site, even for load balancer, so this has been sorted out with EPiServer.

Thanks again for your input.