Comment 5 for bug 740809

well, I solved my case.

I tried to manually run the video export perl script:

sudo -u www-data zmvideo.pl -e 2 -r 1 -s 1 -F 30 -o

and found that script was complaining about permission error when writing ffmpeg.log
revieweing the perl script code I found that ffmpeg.log was being written to the ZM events folder, so:

sudo chown -R www-data:www-data /var/cache/zoneminder
sudo chown -R www-data:www-data /usr/share/zoneminder/events

solved the problem