Okay, I found the issue.
I was using the last stable Avisynth+ version from
https://github.com/AviSynth/AviSynthPlus/releases (3.7.2)
after updating to 3.7.3 r3958 from
https://gitlab.com/uvz/AviSynthPlus-...ng/x64/plugins it seems to work.
So my issue was me, stupidly overlooking that a non-stable, non-official release was a requirement for avs-mrlt.
Cu Selur
Ps.: personally, I'll wait after the next official release before I test this further.