Might be an issue related to framerate, generally it's due to how OS in different languages would use either the dot or the comma, but this issue only used to happen with very old versions of the gui...
Another issue is playback-side: I've found that for some people, playing back mp4s made by zarxgui would result in a sped-up video for whatever reason.
Based on this, I have 2 suggestions:
1) Switch from zarx264gui to some other gui, perhaps one that is updated more often (Alan is lazy and updates it almost never, so the x264 revision is quite old most of the time). One that should be simple is the one done by croma25td
, which has a pretty simple design;
2) Update your playback setup. I suggest using either CCCP