This indicates a possible attack against a Remote Code Execution vulnerability in Microsoft RDP Gateway Server.
The vulnerability is caused by an error when the RDP Gateway Server handles a malicious request. A remote attacker may be able to exploit this to gain control of vulnerable systems via a crafted request.
Windows Server 2019
Windows Server 2016
Windows Server 2012
Windows Server 2012 R2
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.