I stumbled across this topic: ESP32 + SN74AHCT125N, Wiring? Max. length data cable to SK6812? - #5 by Mekanikless
I tried adding several types of resistors between the level shifter and the ledstrip, but that doesn’t make much of a difference for me. I’ve ordered a oscilloscope to find out what exactly causes the problem, so I’ll be waiting for that to arrive.