It has to do with the length of the file name. Names longer than ~8 characters don't work (either in dosbox or this specific program).

Flags work fine but I can't seem to get other parameters to work. Run dosbox >mount c c:\ >c:\program.exe -f textfile.txt doesn't work says can't load textfile.txt >C:\program.exe -f "textfile.txt" doesn't work >c:\program.exe c:\textfile.txt doesnt work >c:\program.exe textfile.txt doesn't work …