[github]

; ========== for MTX testing ================

[External compressor:rz, razor, rz64]
header = 0
default = -d 128m
; packcmd = "Rz\RZ.exe" a {options} $$arcpackedfile$$.tmp $$arcdatafile$$.tmp
; unpackcmd = "Rz\RZ.exe" e -y $$arcpackedfile$$.tmp $$arcdatafile$$.tmp
; or

packcmd = "Rz\RZ.exe" a {options} MyOutput.out MyInput.in
unpackcmd = "Rz\RZ.exe" e -y MyOutput.out MyInput.in
datafile = MyInput.in
packedfile = MyOutput.out

[External compressor:bcm, bcm32]
header = 0
default = -9
; packcmd = "Bcm\{compressor}" {options} $$arcdatafile$$.tmp $$arcpackedfile$$.tmp
; unpackcmd = "Bcm\{compressor}" -d $$arcpackedfile$$.tmp $$arcdatafile$$.tmp
; or

packcmd = "Bcm\{compressor}" {options} MyInput.in MyOutput.out
unpackcmd = "Bcm\{compressor}" -d MyOutput.out MyInput.in
datafile = MyInput.in
packedfile = MyOutput.out

[External compressor:lolz, lolz64]
header = 0
default = -dt -dtb1 -dtw1 -dtm1 -dto1 -dtd1 -mtt1 -mt4 -d64m -fba4096
; packcmd = Lolz\lolz_x64.exe {options} $$arcdatafile$$.tmp $$arcpackedfile$$.tmp
; unpackcmd = Lolz\lolz_x64_u.exe $$arcpackedfile$$.tmp $$arcdatafile$$.tmp
; or

packcmd = Lolz\lolz_x64.exe {options} MyInput.in MyOutput.out
unpackcmd = Lolz\lolz_x64_u.exe MyOutput.out MyInput.in
datafile = MyInput.in
packedfile = MyOutput.out

[External compressor:RazorX, RazorMTX, razorx]
header = 0
default = -c:64m -t:100% --logs
packcmd = "MTX.exe" a:rz {options} - - <stdin> <stdout>
unpackcmd = "MTX.exe" x:rz -t:100p --logs - - <stdin> <stdout>

[External compressor:BcmX, BcmMTX, bcmx]
header = 0
default = -c:64m -t:100% --logs
packcmd = "MTX.exe" a:bcm {options} - - <stdin> <stdout>
unpackcmd = "MTX.exe" x:bcm -t:100p --logs - - <stdin> <stdout>

[External compressor:XLolz, LolzMTX, xlolz]
header = 0
default = -c:64m -t:100% --logs
packcmd = MTX.exe a:lolz {options} - - <stdin> <stdout>
unpackcmd = MTX.exe x:lolz -t:100p --logs - - <stdin> <stdout>

[External compressorrecomp]
header = 0
default = -cn
packcmd = "Precomp\{compressor}" {options} - - <stdin> <stdout>
unpackcmd = "Precomp\{compressor}" -r -o$$arcdatafile$$.tmp $$arcpackedfile$$.tmp
datafile = $$arcdatafile$$.tmp
packedfile = $$arcpackedfile$$.tmp
; or

;packcmd = "Precomp\{compressor}" {options} -oMyOutput.out MyInput.in
;unpackcmd = "Precomp\{compressor}" -r -oMyInput.in MyOutput.out
;datafile = MyInput.in
;packedfile = MyOutput.out

[External compressor:PrecompX, PrecompMTX, precompx]
header = 0
default = -c:64m -t:50% --logs
packcmd = MTX.exe arecomp {options} - - <stdin> <stdout>
unpackcmd = MTX.exe xrecomp -t:100p --logs - - <stdin> <stdout>

I gave the default example and some random files a go:

precomp048+srep+lolz64 works
precomp048+srep+LolzMTX ERROR: general (de)compression error in LolzMTX

precomp048+srep+rz64 works
precomp048+srep+RazorMTX ERROR: general (de)compression error in RazorMTX

precomp048+srep+bcm32 ERROR: general (de)compression error in bcm32
precomp048+srep+BcmMTX works

It is a bit strange that, if not using MTX it works, using MTX will make it not working.
If not using MTX doesn't work, using MTX will make it work?
Is it because of the input output in the settings?

Another test for
PrecompMTX+srep+ any normal or MTX final compressor

none works.

The console / log doesn't say why at all.
It seems as soon as MTX invokes precomp, it exits/crashes.
The temp folder for precomp has been created, only the 0 folder has 512KB file, all others are 0