I've found that Windows KMS clients (2012R2 specifically but may be others) try to talk to MS servers during activation. My understanding was that this should be entirely local.
slmgr /ipk GGWHX-.....-.....-.....-DJ6PQ
slmgr /skms 10.10.10.10:1688
Our servers have very limited access outside our network I've found that the above produces a 0x80072ee2 error unless a proxy server is specified. This has caught me out as I often configure the proxy settings soon after building servers. On this particular server, it is not allowed to have a proxy configured.
Any ideas how to get 2012R2 to activate against an internal KMS server without access to MS via the Internet?