Hi,

Some other application could have taken the port.

If you are on Windows, please also see https://forum.prosysopc.com/forum/opc-ua-simulation-server/prosys-opc-ua-simulation-server-not-running-in-my-local-machine/#p7039 , copied below:
Hyper-V (and possibly some other software) does reserve some ports on Windows.
See https://stackoverflow.com/questions/54010365/how-to-see-what-is-reserving-ephemeral-port-ranges-on-windows see the first 2 answers (in the second one there is “netsh int ip show excludedportrange protocol=tcp”), that should hopefully be able first see that the port was reserved (explains the issue) and then adding it to the excludes causes it to not be reserved anymore (you probably need to restart after adding the port number to the excludes).