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Symptoms:

[ERROR][com.freerdp.core.nego] - Error: SSL_CERT_NOT_ON_SERVER

or

Protocol(warning): Protocol negotiation failed with reason: no valid authentication certificate on server
Retrying with plain RDP.
Core(error): rcp_recv(), connection closed by peer

or

ERRCONNECT_SECURITY_NEGO_CONNECT_FAILED [0x0002000C]

Solution:

VBoxManage modifyvm "myVM" --vrdeproperty "Security/Method=Negotiate"