After upgrading a site, "Server is not compatible" is shown when accessing Web Reports.
This message is usually shown when the client (Workstation or Web Reports) used is not the same version as the server (AS or ES) you are accessing. However, in some cases after upgrading a site, this message can be seen even though the server you are connecting to is in fact the same version as the client.
This is because the version of the client is cached in the server.
As described in the upgrade guide, the Web Reports agent should be stopped during upgrade of the ES. If the agent was not stopped during the upgrade, you can get in a state where this message is shown.
The resolution is to simply restart the ES.