I believe this is a small bug:
[main] TRACE 00:45:29.720 Found AviSynth plugins dir: C:\Program Files\AviSynth 2.5\plugins
[main] TRACE 00:45:29.720 !!! It seems VSFilter.dll is not installed into the AviSynth plugins dir ! It could be troublesome for playing subtitled videos with AviSynth !!!
Because VSFilter.dll is installed into the AviSynth plugins dir, namely at:
C:\Program Files\AviSynth 2.5\plugins'\VSFilter\VSFilter.dll
It was easy enough, of course, to just copy VSFilter.dll to the parent dir, but perhaps this small bug can be fixed in the upcoming version?