Find out what holds the port
v2rayN listens on 10808 for SOCKS and 10809 for HTTP by default. In PowerShell run netstat -ano | findstr 10808; the last column is the PID. Match it in Task Manager under the Details tab. The usual suspect is an old v2rayN that never shut down, or a second proxy client.
Move it to another port
Change the local listening port under Settings → Parameter settings — 20808 works fine — then restart the core. The system proxy follows automatically, but if you typed the port into a browser extension by hand, update that too.
New port, same failure
- Ports below 1024 need administrator rights. Don't use 80 or 443.
- Hyper-V and WSL reserve a block of dynamic ports, and anything inside that range is held by the system. Moving above 20000 normally clears it.