ffmpegthumbnailer: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/ffmpegthumbnailer - from package: ffmpegthumbnailer

You may install the required package with command apt-get install {package}

Error in other languages:
ffmpegthumbnailer: bevel nie gevind nie
ffmpegthumbnailer: comandă negăsită
ffmpegthumbnailer: comando não encontrado
ffmpegthumbnailer: commande introuvable
ffmpegthumbnailer: command not found
ffmpegthumbnailer: käsku ei ole
ffmpegthumbnailer: không tìm thấy lệnh
ffmpegthumbnailer: komanda nerasta
ffmpegthumbnailer: Kommando nicht gefunden.
ffmpegthumbnailer: kommandot finns inte
ffmpegthumbnailer: komut yok
ffmpegthumbnailer: nie znaleziono polecenia
ffmpegthumbnailer: níor aimsíodh an t-ordú
ffmpegthumbnailer: no se encontró la orden
ffmpegthumbnailer: no s'ha trobat l'ordre
ffmpegthumbnailer: opdracht niet gevonden
ffmpegthumbnailer: parancs nem található
ffmpegthumbnailer: perintah tidak ditemukan
ffmpegthumbnailer: príkaz nenájdený
ffmpegthumbnailer: příkaz nenalezen
ffmpegthumbnailer: команда не найдена
ffmpegthumbnailer: командата не е открита
ffmpegthumbnailer: 命令找不到
ffmpegthumbnailer: コマンドが見つかりません