Customers had started to report that ipconfig /flushdns now requires Local Administrator rights.
The requested operation requires elevation.
Not all customers are seeing that though.
(Microsoft has gone and changed something. Not sure what yet.)
Workaround using PowerShell
Clear-DNSClientCache command will do this instead.
Clears the contents of the DNS client cache.
Clear-DnsClientCache [-CimSession <CimSession>] [-ThrottleLimit <Int32>] [-Confirm] [-WhatIf]
The Clear-DnsClientCache cmdlet deletes all the contents of the DNS client cache. Running this cmdlet is
equivalent to running ipconfig /flushdns.