I had this issue aswell on my windows machine (develop). I tried to change the port in config/server.js but strapi admin panel still sent requests to port 1337. Therefore the login to the admin panel was not possible for me with this solution.
My solution to this problem was to figure out the process id with the command “netstat -ano | findstr :1337” in command prompt. With the output process id I went to the task manager and watched under the details tab which application corresponds to the process id. In my case it was the Razer Chroma app. After I uninstalled the razer App, I had no more issues with the port 1337 and could use strapi without problems 