In Big Sur 11.2, I’m on Local 5.9.9, just re-installed yesterday. Every time I reboot and start a site, I get a port conflict message as well. I see the 502 Gateway error.
I open terminal and run the suggested command. Kill the process running as root for nginx, using the suggested command on the help page you mentioned. There are several other running on port 80, but with my user account.
After that, the site will run after I restart it in Local. Pretty annoying to kill that task every time. I couldn’t find any clues as to why that task is running and how to start it at startup.
MAMP was installed, but I used AppCleaner to remove it. Is it the Local.app running on port 80 or some other MacOS process.