LoRd_MuldeR pointed me at it, and I can only confirm:
The x264 builds created by media-autobuild_suite including FFMS2 and L-SMASH Source (video codecs only = mode 6/"fullv") can't open AVI sources and eventually fall back to AviSynth, trying to use AviSource.
Code:
x264 -o *.mp4 *.avi
ffms [error]: could not create indexer
lavf [error]: could not open input file
avs [info]: trying AVISource... succeeded
...
I will try to create a full build (mode 4/"full"), wondering if possibly the limiting to video codecs only may cause issues. Could anyone else with enough insight possibly check if MABS might be misconfigured for this case? Or is it possible that there is indeed a mistake in current x264 sources?
_
In the meantime,
issue #758 appeared, and
commit c566f85; will build again...