PORTB:
It looks like RCSwitch uses interrupts to receive data. The WS2812b leds need precise timing, and therefore the neopixel library disables interrupts, so anything using interrupts probably won't work.
I tried disabling the receive pin before setting the LEDs and enabling it afterwards but that didn't help. Enabling interrupts manually doesn't do anything either. I changed the rc-switch interrupt handling so it prints something everytime an interrupt occurs. That worked even after receiving the first data. The Arduino somehow just doesn't recognize what's been sent.
Using different LED libraries (FastLED and light_ws2812) has the same result.
DrAzzy:
I suspect a bug in your code
I hope it's in in the code, then I can fix it.
Thanks for the replies so far!