You need to make sure you only run sites which have licenses for the number of servers that are configured for your current Kentico CMS instance. An example of when you could run into a problem – you have web farm environment enabled and 2 web farm servers running. There are 2 sites running on the instance – one has a license key for 2 servers, the other only for 1 server. If you try to access the site with a license for only 1 server, this action will always result in logging the error because system will try to balance the load to the second web farm server for which you do not have a valid license.
Solution is to make a copy of the database and run the sites on separate Kentico CMS instances from which only one would have web farm environment configured – the one with the site and license for 2 servers.