Wemos D1 mini won't connect to WiFi: status code 6
Apparently, something went wrong with connecting to the router. The router was casting 802.11 g + n, which for some reason, made the esp8266 give a code 6. I have tried 802.11 b, 802.11 b+g+n, 802.11 n, which work all fine.
So I updated the settings that the router now uses 802.11n and the esp8266 can now connect to the internet!