1) Are you running Windows or Linux (or XAMPP)?
2) If you are running Linux, then your FFmpeg binary is in a (very) non-standard location. How and who installed FFmpeg for you, and are you perhaps using a standalone version of FFmpeg? If you type 'which ffmpeg' at the command line, what is the output? If you type 'ffmpeg' in the command line, what is the output?
3) You are using a combination of relative and absolute file paths to assign your constant values. Is there a reason for this? Perhaps you could try all relative paths or all absolute paths, instead of a mix of both?
4) Are you using the free or commercial version of the software? Both versions require the libmp3lame codec to be installed, and the paid-for version additionally requires libx264 and aac codecs.
In terms of troubleshooting, please try the following:
1) Please check that the _FFMPEG constant value in config.class.php points to the correct location of the FFmpeg binary on your Linux server.
2) Please ensure that all relevant directories and files are chmod to 0777
3) Please check your ‘videos’ directory. Are there video files in the directory? If so, are the video files playable? Are the videos fully downloaded? Are the videos 0 kb in length?
4) Please check your ‘logs’ directory for FFmpeg log files. Are there any log files in the directory? If so, please copy and paste the contents of a log file that corresponds to a failed conversion. (There should be errors in the log file.)