mp4 conversion doesn't work on debian lenny

I tried to download a youtube video and to convert it in mp4 to read it into my cellphone but when I convert it in mp4 the conversion end in 0 seconds with "DONE" message but no video Other conversions work correctly. I installed pytube 0.0.10.8 on debian lenny from official repository.

If I start pytube from the terminal and try to convert "prova.avi" I have this output:

clo7@koi:~$ pytube
./pytube.py:568: GtkWarning: gtk_tree_model_get_n_columns: assertion `GTK_IS_TREE_MODEL (tree_model)' failed
  self.wTree = gtk.glade.XML(self.gladefile, "mainwindow")
./pytube.py:568: GtkWarning: gtk_combo_box_set_row_span_column: assertion `row_span >= -1 && row_span < col' failed
  self.wTree = gtk.glade.XML(self.gladefile, "mainwindow")
./pytube.py:568: GtkWarning: gtk_combo_box_set_column_span_column: assertion `column_span >= -1 && column_span < col' failed
  self.wTree = gtk.glade.XML(self.gladefile, "mainwindow")
./pytube.py:568: GtkWarning: gtk_tree_row_reference_new: assertion `GTK_IS_TREE_MODEL (model)' failed
  self.wTree = gtk.glade.XML(self.gladefile, "mainwindow")
./pytube.py:568: GtkWarning: gtk_cell_view_set_displayed_row: assertion `GTK_IS_TREE_MODEL (cell_view->priv->model)' failed
  self.wTree = gtk.glade.XML(self.gladefile, "mainwindow")
./pytube.py:568: GtkWarning: gtk_radio_button_set_group: assertion `!g_slist_find (group, radio_button)' failed
  self.wTree = gtk.glade.XML(self.gladefile, "mainwindow")
Configuration file found
Latest version: 0.0.10.8
Current version: 0.0.10.8
prova.avi
FFmpeg version SVN-r12810, Copyright (c) 2000-2008 Fabrice Bellard, et al.
  configuration: --prefix=/usr --libdir=${prefix}/lib --shlibdir=${prefix}/lib --incdir=${prefix}/include/ffmpeg --enable-shared --enable-libmp3lame --enable-gpl --enable-libfaad --mandir=${prefix}/share/man --enable-libvorbis --enable-pthreads --enable-libfaac --enable-libxvid --enable-postproc --enable-libamr-nb --enable-libamr-wb --enable-x11grab --enable-libgsm --enable-libx264 --enable-liba52 --enable-libtheora --extra-cflags=-Wall -g -fPIC -DPIC --cc=ccache cc --enable-swscale --enable-libdc1394 --enable-nonfree --disable-mmx
  libavutil version: 49.6.0
  libavcodec version: 51.54.0
  libavformat version: 52.13.0
  libavdevice version: 52.0.0
  built on Apr 14 2008 17:21:37, gcc: 4.2.3 (Debian 4.2.3-3)
Input #0, avi, from '/home/clo7/prova.avi':
  Duration: 00:06:01.1, start: 0.000000, bitrate: 262 kb/s
    Stream #0.0: Video: mpeg4, yuv420p, 320x240 [PAR 1:1 DAR 4:3], 12.00 tb(r)
    Stream #0.1: Audio: mp2, 22050 Hz, mono, 64 kb/s
Output #0, image2, to '/home/clo7/.pytube_framepic.png':
    Stream #0.0: Video: png, rgb24, 130x98 [PAR 196:195 DAR 4:3], q=2-31, 200 kb/s, 12.00 tb(c)
Stream mapping:
  Stream #0.0 -> #0.0
Press [q] to stop encoding
frame= 1 fps= 0 q=0.0 Lsize= -0kB time=0.1 bitrate= -2.1kbits/s
video:1kB audio:0kB global headers:0kB muxing overhead -101.819686%
FFmpeg version SVN-r12810, Copyright (c) 2000-2008 Fabrice Bellard, et al.
  configuration: --prefix=/usr --libdir=${prefix}/lib --shlibdir=${prefix}/lib --incdir=${prefix}/include/ffmpeg --enable-shared --enable-libmp3lame --enable-gpl --enable-libfaad --mandir=${prefix}/share/man --enable-libvorbis --enable-pthreads --enable-libfaac --enable-libxvid --enable-postproc --enable-libamr-nb --enable-libamr-wb --enable-x11grab --enable-libgsm --enable-libx264 --enable-liba52 --enable-libtheora --extra-cflags=-Wall -g -fPIC -DPIC --cc=ccache cc --enable-swscale --enable-libdc1394 --enable-nonfree --disable-mmx
  libavutil version: 49.6.0
  libavcodec version: 51.54.0
  libavformat version: 52.13.0
  libavdevice version: 52.0.0
  built on Apr 14 2008 17:21:37, gcc: 4.2.3 (Debian 4.2.3-3)
Input #0, avi, from '/home/clo7/prova.avi':
  Duration: 00:06:01.1, start: 0.000000, bitrate: 262 kb/s
    Stream #0.0: Video: mpeg4, yuv420p, 320x240 [PAR 1:1 DAR 4:3], 12.00 tb(r)
    Stream #0.1: Audio: mp2, 22050 Hz, mono, 64 kb/s
WARNING: The bitrate parameter is set too low. It takes bits/s as argument, not kbits/s
Unknown encoder 'aac'
/home/clo7/prova.avi
/home/clo7/prova

Thanx