I am having similar issues on the develop path. I had to change back to the master branch and it went back to working.

My error looks like:

mm-0 [18:41:00.356] [ERROR]
mm-0 WARNING! Could not load config file. Starting with default configuration. Error found: Error: Cannot find module 'node_helper'
mm-0 Require stack:
mm-0 - /home/pi/MagicMirror/modules/MMM-Remote-Control/node_helper.js
mm-0 - /home/pi/MagicMirror/js/app.js
mm-0 - /home/pi/MagicMirror/js/electron.js
mm-0 - /home/pi/MagicMirror/node_modules/electron/dist/resources/default_app.asar/main.js

@morozgrafix So I tried changing the port number to 8081 and I got this:

mm-0 (err): WARNING! Could not load config file. Starting with default configuration. Error found: RangeError: "port" argument must be >= 0 and < 65536
mm-0 (err): App threw an error during load
mm-0 (err): RangeError: "port" argument must be >= 0 and < 65536
mm-0 (err):     at assertPort (internal/net.js:17:11)
mm-0 (err):     at Server.listen (net.js:1371:7)
mm-0 (err):     at new Server (/home/pi/MagicMirror/js/server.js:20:9)
mm-0 (err):     at /home/pi/MagicMirror/js/app.js:193:18
mm-0 (err):     at loadNextModule (/home/pi/MagicMirror/js/app.js:142:5)
mm-0 (err):     at /home/pi/MagicMirror/js/app.js:137:6
mm-0 (err):     at Class.loaded (/home/pi/MagicMirror/modules/node_modules/node_helper/index.js:19:3)
mm-0 (err):     at loadModule (/home/pi/MagicMirror/js/app.js:118:6)
mm-0 (err):     at loadNextModule (/home/pi/MagicMirror/js/app.js:135:5)
mm-0 (err):     at /home/pi/MagicMirror/js/app.js:137:6
mm-0 (err): ERROR! Could not find main module js file.

I did run npm install, also tried serveronly and running npm start dev but did not gather any additional information. I will say the weird thing is when I leave it on port 8080, the remote control module still loads.

@morozgrafix maybe, I issued a pm2 stop and then did a restart and I am no longer getting that error. I am still getting a black only screen with the only error being:

"ERROR! Could not find main module js file."

What logs should I get you to figure out the reason for lack of display?

Edit:
Scratch that, I double checked the logs and I get the same error. I will do a full pi reboot again to triple check but it seems consistent.
Edit 2:
After triple check, only one MM instance open and same error noted about port 8080.

Something on the develop branch turned my magic mirror back to black screen again (It ran fine on develop branch yesterday but when I did my pull&&install today it went down again). Here is the information from the pm2 logs, tell me if you need anything else:

mm-0 (out): Module helper loaded: MMM-NFL
mm-0 (out): Initializing new module helper ...
mm-0 (out): Module helper loaded: newsfeed
mm-0 (out): All module helpers loaded.
mm-0 (out): Starting server op port 8080 ...
mm-0 (out): Server started ...
mm-0 (out): Connecting socket for: MMM-Remote-Control
mm-0 (out): Starting node helper for: MMM-Remote-Control
mm-0 (out): Connecting socket for: calendar
mm-0 (out): Starting node helper for: calendar
mm-0 (out): Connecting socket for: MMM-Traffic
mm-0 (out): MMM-Traffic helper started ...
mm-0 (out): Connecting socket for: MMM-WunderGround
mm-0 (out): MMM-WunderGround helper started ...
mm-0 (out): Connecting socket for: MMM-NFL
mm-0 (out): Starting module: MMM-NFL
mm-0 (out): Connecting socket for: newsfeed
mm-0 (out): Starting module: newsfeed
mm-0 (out): Sockets connected & modules started ...
mm-0 (out): Launching application.

mm-0 (err):     at exports._exceptionWithHostPort (util.js:1049:20)
mm-0 (err):     at Server._listen2 (net.js:1253:14)
mm-0 (err):     at listen (net.js:1289:10)
mm-0 (err):     at net.js:1399:9
mm-0 (err):     at _combinedTickCallback (internal/process/next_tick.js:77:11)
mm-0 (err):     at process._tickCallback (internal/process/next_tick.js:98:9)
mm-0 (err):     at Module.runMain (module.js:592:11)
mm-0 (err):     at run (bootstrap_node.js:402:7)
mm-0 (err):     at startup (bootstrap_node.js:157:9)
mm-0 (err):   code: 'EADDRINUSE',
mm-0 (err):   errno: 'EADDRINUSE',
mm-0 (err):   syscall: 'listen',
mm-0 (err):   address: '::',
mm-0 (err):   port: 8080 }
mm-0 (err): ERROR! Could not find main module js file.

@MichMich I am on the latest develop and it throws an error and just gives me a black screen:

> magicmirror@2.1.1 start /home/pi/MagicMirror
> sh run-start.sh

Starting MagicMirror: v2.1.1
Loading config ...
App threw an error during load
TypeError: Path must be a string. Received undefined
    at assertPath (path.js:7:11)
    at Object.resolve (path.js:1148:7)
    at loadConfig (/home/pi/MagicMirror/js/app.js:53:29)
    at App.start (/home/pi/MagicMirror/js/app.js:180:3)
    at Object. (/home/pi/MagicMirror/js/electron.js:102:6)
    at Module._compile (module.js:556:32)
    at Object.Module._extensions..js (module.js:565:10)
    at Module.load (module.js:473:32)
    at tryModuleLoad (module.js:432:12)
    at Function.Module._load (module.js:424:3)
Whoops! There was an uncaught exception...
TypeError: Path must be a string. Received undefined
    at assertPath (path.js:7:11)
    at Object.resolve (path.js:1148:7)
    at loadConfig (/home/pi/MagicMirror/js/app.js:53:29)
    at App.start (/home/pi/MagicMirror/js/app.js:180:3)
    at Object. (/home/pi/MagicMirror/js/electron.js:102:6)
    at Module._compile (module.js:556:32)
    at Object.Module._extensions..js (module.js:565:10)
    at Module.load (module.js:473:32)
    at tryModuleLoad (module.js:432:12)
    at Function.Module._load (module.js:424:3)
MagicMirror will not quit, but it might be a good idea to check why this happened. Maybe no internet connection?
If you think this really is an issue, please open an issue on GitHub: https://github.com/MichMich/MagicMirror/issues
Launching application.

When I jump back to Master branch it loads fine:

//your code> magicmirror@2.1.0 start /home/pi/MagicMirror
> electron js/electron.js

Starting MagicMirror: v2.1.0
Loading config ...
Loading module helpers ...
No helper found for module: alert.
Initializing new module helper ...
No helper found for module: clock.
Initializing new module helper ...
Initializing new module helper ...
Initializing new module helper ...
Initializing new module helper ...
Initializing new module helper ...
All module helpers loaded.
Starting server op port 8080 ... 
Server started ...
Connecting socket for: MMM-Remote-Control
Starting node helper for: MMM-Remote-Control
Connecting socket for: calendar
Starting node helper for: calendar
Connecting socket for: MMM-Traffic
MMM-Traffic helper started ...
Connecting socket for: MMM-WunderGround
MMM-WunderGround helper started ...
Connecting socket for: MMM-NFL
Starting module: MMM-NFL
Connecting socket for: newsfeed
Starting module: newsfeed
Sockets connected & modules started ...
Launching application.
ERROR! Could not find main module js