It’s basically a fresh install. I bought a new EMMC (32GB, it came with an 8GB one) and did everything from scratch following this guide:
The randomness of it makes me believe that it might not actually be a software issue. Could also be memory or maybe even a lose connection somewhere.
Today it’s also printing just fine after I had taken down the WiFi interface (wlan0). That trick also did it yesterday. Maybe it’s the WiFi module? One day it crashed when I was uploading prints.