i think you need specificate decoding/encoding streams option (-t#), try this config.
[External compressor
zlib]
header = 0
packcmd = pZLib e -m2 -x -c256m -t75p $$arcdatafile$$.tmp -o $$arcpackedfile$$.tmp
unpackcmd = pZLib d -t75p - -o - <stdin> <stdout>