System: Windows 8 x64
running asy.exe I get
Welcome to Asymptote version 2.21 (to view the manual, type help)
error: could not load module 'plain'
Meantime I was able to eliminate the error by adding
os.setenv('ASYMPTOTE_HOME', TEXDIR..'/texmf-dist/asymptote')
to runscript.tlu
But I don't think it is the proper solution