Ok, i make it short. The game supports runing a replay from outside the game via command line. But if the command line contains other characters then US-ASCII, you'll get a error message and it says its unable to load the file.
So, if your OS login name is something like "André" (C:\Documents & Settings\André\...) or the filename itself contain any non-US-ASCII characters, then your are not able to run a replay from outside the game. It's still possible to start it from within the game, so i think its a problem in converting the filename to the correct representation expected by the lua-interpreter.
Example filenames: "C:\andré.FRGReplay" or "C:\jörg.FRGReplay"
I hope the devs know what i mean and hopefully they can fix it.
Thanks.