Get-LocalGroupMember generates error for Administrators group
This has been referenced as an official bug:
https://github.com/PowerShell/PowerShell/issues/2996
Here is workaround:
http://jdhitsolutions.com/blog/scripting/2342/query-local-administrators-with-cim/
UPDATE:
I had some issues with CIM and WMI.
Here is another workaround which worked everywhere for me.
https://p0w3rsh3ll.wordpress.com/2016/06/14/any-documented-adsi-changes-in-powershell-5-0/