Hello,
After some research & test, here is my fix :
1/
In C:\Users\yourname\AppData\Local\UnrealEngine\5.1\Config
Add :
[/Script/UdpMessaging.UdpMessagingSettings]
EnabledByDefault=False
EnableTransport=True
bAutoRepair=True
MaxSendRate=1.000000
AutoRepairAttemptLimit=10
WorkQueueSize=1024
bStopServiceWhenAppDeactivates=True
UnicastEndpoint=0.0.0.0:0
MulticastEndpoint=230.0.0.1:50000
MessageFormat=CborPlatformEndianness
MulticastTimeToLive=1
EnableTunnel=False
TunnelUnicastEndpoint=
TunnelMulticastEndpoint=
To the file DefaultEngine.ini
2/ Remove Engine.ini in C:\Users\yourname\AppData\Local\UnrealEngine\5.1\Saved\Config\WindowsEditor
When you will launch UE again, the Engine.ini will be regenerated with the port 50000 instead of 6666 for the parameter MulticastEndpoint. It should allow you to launch UE without crashing.