the libffmpeg 0.55.0 not have an av_stream_get_first_dts symbol. So, this's the alternative one: https://github.com/Ld-Hagen/fix-opera-linux-ffmpeg-widevine/releases
You also get it from Vivaldi browser, but I'm not sure it work in slimjet 33 (see more here): https://github.com/iteufel/nwjs-ffmpeg-prebuilt/issues/119