It sounds like something on your Pi is attempting to start the Magic Mirror process twice. If you used a cron job to start MM as well as PM2, then this can happen. Likewise, if your script that PM2 calls to start the process has the command twice, this can happen.
EADDRINUSE usually means that two processes are trying to serve up web pages on the same port. If that happens on reboot, then most likely two commands are trying to start MM.
My recommendation is to remove PM2’s attempts to start MM, reboot and see if the mirror process starts. If so, find out how it’s being called and remove that, then reset pm2 to do it. If MM doesn’t start after the reboot, run your netstat -lptn again and find out what’s listening on that port.