I would like to inform you that ePO 5.3.3 and later does not support Windows Server 2008.
If you are running Windows Storage Server 2008 R2 (Standard, Datacenter, Enterprise, and Web) then you can upgrade the ENS for any of these versions ENS 10.5.0-10.5.2 ENS 10.5.3 ENS 10.5.4, 10.6.0 and ENS 10.5.5, 10.6.1
If you are just running Windows Server 2008, then I would suggest you to upgrade the ePO console to the 5.9.1 and continue using 10.2 for the Windows Server 2008 machines.
KB51109 is the master article for supported environments for the products. In that kb there are links to the various products. From there, you can find what versions of our products are supported on what OS. Keep in mind that most of the product extensions are backwards compatible, so a 10.6 ens extension would manage a 10.2 ens client. When looking at compatibility for products with different versions of epo, it is mainly only referring to the extension versions.
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