I try to start it directly from the prompt

npm install electron --save-dev

electron@1.4.12 postinstall /home/pi/MagicMirror/node_modules/electron
node install.js

magicmirror@2.0.0 /home/pi/MagicMirror
├── electron@1.4.12
└── UNMET PEER DEPENDENCY grunt@1.0.1

npm WARN grunt-yamllint@0.2.0 requires a peer of grunt@~0.4.0 but none was installed.

Yep,
deleteing as well as renaming the MagicMirror Folder won’t do the trick for me.
Maybe it’s related to this Issue https://github.com/MichMich/MagicMirror/issues/567

Where should the electron.js be located, i believe it’s missing here too.

I’ve got the same Problem.
Fresh installation of Raspian on an Raspberry Pi3.

Running the install Script again doesn’t help.