I was able to trace the problem back to the Adafruit NeoPixel library, which apparently is being used by the Ws2812FX-library to actually generate the data stream. Once I was on the Adafruit GitHub I found several issue-reports regarding the same problem I'm having. Some incompatibility between ESP32 Arduino Core 3 and the Adafruit library, indeed regarding buffer allocation. Reverting both Adafruit-library and the Arduino Core results in a stable build, which is fine for me until Adafruit updates the library.