The solution is very simple: Windows transfers the full path automatically with double quotation marks into the replaceable parameters (%1, %2, ...) and IrfanView has a big problem with that. Delete this marks and it works great.
Example:
@echo off
cls
setlocal
:: --- Configuration --->
:: Path to IrfanView
set iview=C:\IrfanView\i_view32.exe
:: Destination path for multipage tif
set tifpath=C:\Meine Bilder
:: File name for multi tif picture
set tifname=multitif.tif
:: <--- Configuration ---
"%iview%" /multitif=(%tifpath%\%tifname%,%~1,%~2) /killmesoftly
endlocal