just found out it depends what container the source is.
transcoding of a testvideo in mp4 container works but when this video is put in a mkv container via MKVToolNixGUI it fails.
log of the mkv-version of the working mp4 video
20:48:40.671: ===== Starting mpv ======
20:48:40.671: Command line: C:\Program Files (x86)\SVP 4\mpv64\mpv.exe I:/testvid-2.mkv --o=C:/output/testvid-2.SVP.temporary.mkv --no-audio --no-sub --no-sub-auto --input-ipc-server=mpvencodepipe --input-media-keys=no --no-msg-color --vf=vapoursynth:[C:\Users\user.DESKTOP-4VN3MOC\AppData\Roaming\SVP4\scripts\ffff.py]:4:4,fps=59.940 --of=matroska --ovc=hevc_qsv --ovcopts=preset=veryslow,look_ahead=1,global_quality=21,time_base=1000/59940,threads=4
20:48:40.797: (+) Video --vid=1 (*) (h264 960x540 29.970fps)
20:48:40.797: Audio --aid=1 (*) (aac 2ch 48000Hz)
20:48:47.766: [autoconvert] Converting yuv420p -> nv12
20:48:47.773: VO: [lavc] 960x540 nv12
20:48:47.773: [vo/lavc] Opening encoder: HEVC (Intel Quick Sync Video acceleration) [hevc_qsv]
20:48:47.774: [vo/lavc] AVOption 'look_ahead' not found.
20:48:48.137: [encode] Opening output file: C:/output/testvid-2.SVP.temporary.mkv
20:48:48.137: [encode] Opening muxer: Matroska [matroska]
20:48:48.150: V: 00:00:00 / 00:15:24 (0%) {0.3min 0.0MB} Cache: 923s/97MB
20:48:48.471: [ffmpeg] matroska: Application provided invalid, non monotonically increasing dts to muxer in stream 0: 150 >= 133
20:48:48.471: [encode] Writing packet failed.
20:48:48.767: Exiting... (Interrupted by error)
20:48:48.767: V: 00:00:00 / 00:15:24 (0%) {7.7min 18.2fps 0.0MB} Cache: 923s/97MB
20:48:48.871: (!!!) Intermediate file may be broken: C:\output\testvid-2.SVP.temporary.mkv
20:48:48.871: ===== mpv exited with code 2 =====