Esync allows you to use an Ektron Windows Service (EWS) version that is greater than the version of the sites on the server. There are a few version rules which should be followed.
The Ektron Windows Service (EWS) version should be the same version between syncing servers. This is present in the following file:
C:\Program Files (x86)\Ektron\EktronWindowsService40\Ektron.ASM.EktronServices40.exe.config
For the version find the following key.
= 9.1 (220.127.116.11) SP1 (.1) CU 4 (.304)
The EWS version should be greater than or equal to the latest site version. This is found in the web.config.
The sites being synced should be on the same service pack and cumulative site update(CU). Look at the following values.