If video files that previously worked are not working now, especially .mp4 files, there is likely a problem with the codecs installed on the system. Such problems occur most often when other software using video is installed, uninstalled, or updated. You may receive Media File Error (Id 8356) in EthoVision indicating such a problem. There is a tool available which can resolve many such codec problems.
If you have the EthoVision XT 16 or 17 installation USB stick, this tool can be found in the folder \Drivers and tools\Utilities\MediaTypeRegistryCleaner Otherwise, download this file: https://noldus.sharefile.eu/d-s23c8e2c4233c4e518dcdf773ec402360 If you have downloaded the zip file, unzip the file by right-clicking it and picking Extract All.
To use the tool, in the x64 folder, right-click the MediaTypeRegistryCleaner file and pick "Run as administrator" (if you do not have administrative access on your PC, you may need the assistance of your local IT department).
Reboot the computer and check whether the issue is resolved.