If a video file cannot be played, locale FFmpeg executable (in lib folder in Serviio installation directory, or a part of your Linux distribution), run this command and post the result in the forum:
ffmpeg.exe -i "<your_file_location>"
On Linux/ Mac:
ffmpeg -i <your_file_location>
Sometimes, it is also useful to provide output of MediaInfo (especially for H264 MOV/MP4/MKV files)
Also make sure you mention your OS, renderer device and we might ask you for a detailed log as well.