Found it! It's the add-startup script located in the wbstartup drawer that comes Pack Chrysalis.

The first lines include:

;BEGIN Arexx
assign >NIL: MUIRexx: c:
If Exists mossys:libs/rexxsyslib.library
rename Mossys:Libs/rexxsyslib.library Mossys:Libs/rexxsyslib.library_disabled
EndIf
;END Arexx

Should I just delete them? I think that I need the script because it has other various assigns regarding FreeDB, AmiSSL and more.