Video – FFMPEG (Synology): Error opening the encoder for the output stream # 0: 0 – possibly wrong parameters like bitrate, rate, width or height

I'm trying to convert my AVI files to mp4 so Plex does not have to recode them.
I want to run it from my Synology NAS where ffmpeg is installed.
When I execute this command ffmpeg -i Pilot.avi Pilot.mp4 or a variant I get:

ffmpeg version 2.7.1 Copyright (c) 2000-2015 the FFmpeg developers built with gcc 4.9.4 (crosstool-NG crosstool-ng-1.22.0-169-g87eb166) 20150629 (prerelease)
configuration: --prefix=/usr --incdir='${prefix}/include/ffmpeg' --arch=arm64 --target-os=linux --cross-prefix=/usr/local/aarch64-unknown-linux-gnueabi/bin/aarch64-unknown-linux-gnueabi- --enable-cross-compile --enable-optimizations --enable-pic --enable-gpl --enable-shared --disable-static --enable-version3 --enable-nonfree --enable-libfaac --enable-encoders --enable-pthreads --disable-bzlib --disable-protocol=rtp --disable-muxer=image2 --disable-muxer=image2pipe --disable-swscale-alpha --disable-ffserver --disable-ffplay --disable-devices --disable-bzlib --disable-altivec --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libmp3lame --disable-vaapi --disable-decoder=amrnb --disable-decoder=ac3 --disable-decoder=ac3_fixed --disable-encoder=zmbv --disable-encoder=dca --disable-encoder=ac3 --disable-encoder=ac3_fixed --disable-encoder=eac3 --disable-decoder=dca --disable-decoder=eac3 --disable-decoder=truehd --cc=/usr/local/aarch64-unknown-linux-gnueabi/bin/aarch64-unknown-linux-gnueabi-ccache-gcc --enable-yasm --enable-libx264 --enable-encoder=libx264
libavutil 54. 27.100 / 54. 27.100
libavcodec 56. 41.100 / 56. 41.100
libavformat 56. 36.100 / 56. 36.100
libavdevice 56. 4.100 / 56. 4.100
libavfilter 5. 16.101 / 5. 16.101
libswscale 3. 1.101 / 3. 1.101
libswresample 1. 2.100 / 1. 2.100
libpostproc 53. 3.100 / 53. 3.100
Input #0, avi, from 'Pilot.avi':
Metadata:
encoder : VirtualDubMod 1.5.4.1 (build 2178/release)
IAS1 : English
Duration: 00:36:26.00, start: 0.000000, bitrate: 893 kb/s
Stream #0:0: Video: mpeg4 (Advanced Simple Profile) (XVID / 0x44495658), yuv420p, 512x384 (SAR 1:1 DAR 4:3), 779 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc
Stream #0:1: Audio: mp3 (U(0)(0)(0) / 0x0055), 48000 Hz, stereo, s16p, 101 kb/s
Output #0, mp4, to 'Pilot.mp4':
Metadata:
encoder : VirtualDubMod 1.5.4.1 (build 2178/release)
IAS1 : English
Stream #0:0: Video: h264, none, q=2-31, 128 kb/s, SAR 1:1 DAR 0:0, 25 fps
Metadata:
encoder : Lavc56.41.100 libx264
Stream #0:1: Audio: aac, 0 channels, 128 kb/s
Metadata:
encoder : Lavc56.41.100 libfaac
Stream mapping:
Stream #0:0 -> #0:0 (mpeg4 (native) -> h264 (libx264))
Stream #0:1 -> #0:1 (mp3 (native) -> aac (libfaac))
Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height

What's wrong? What can I do to correct it?

Thank you very much