Hi Vanessa, I don't know why that video is giving you problems but I suggest you try converting it to a different format and see if that works.  We have had good results using the open source application HandBrake, available for Windows, Linux and MacOS platforms at https://handbrake.fr/. Choose the default "Fast 1080p30" preset, then in the Video tab choose "Constant Framerate". If you cannot or do not want to install software on your computer, you can use a web-based converter such as Cloud Convert.

You mentioned that your video may not have a constant frame rate. If so, you will not be able to get reliable motion data from a point mass track, as the velocity and acceleration algorithms used assume a fixed time interval between frames.

>
>  I import
> a video as .mp4 video (Codec: H264-MPEG-4 AVC (part10)(avc1),
> Dimensions: 720 x 540, 29,7 fps (not constant)) to
> Tracker Online (no matter whether I use drag and drop
> or open file). I loaded another video with the same
> codec, dimensions and fps, only extended video length
> and it worked. If I open the video with VLC Player
> it runs normally...
> The video appears in the  little
> preview window and then loads in the tracker interface.
> So far so good but wen I press play the video starts
> the first 3 frames and stops afterwards while the
> time bar is going further.
> I tried to change the
> video format in different formats but the video cannot
> be opened or the same problem occurs.
>
> Can someone
> here help me to fix this?
>
> Thanks in advance,
>
> Vanessa