So I feel slightly stupid, but for anyone in a similar situation, @sdetweil helped @JorickJuh in this thread with the same problem. I don’t know how to hotlink replies, so for anyone wondering, @sdetweil suggested that, as is evident from the log, node_helper is missing. So, for me, this did the trick:
cd ~/MagicMirror
git checkout modules/node_modules/node_helper/index.js
EDIT: you beat me to it :D - thanks again!