Jacky,
I have just tired to open my test mp4 file in Walker controller on Windows 10 and CLAN worked as expected. My best suggestion without knowing the System your RA is using or knowing anything about mp4 file or knowing whether CLAN crashes or just simply does not open mp4 media window is to make sure that Walker Controller's window settings are set correctly. I would suggest to start with "Walk length" = 4000, "Loop number" = 1, "Backspace" = 500, "Walk pause len" = 0, "Playback speed" = 100. If this fixes the problem, then you can change setting to something else.
If this doesn't work, then make sure that the computer system is updated and try rebooting computer. If it still doesn't work, then I need more information about the system and what exactly is happing and at what step of using Walker Controller. We might even need to get the mp4 file for testing is every else fails.