FFMPEG error while converting flv1 (flv) -> h264 (libx264) - ffmpeg

Question

I am trying to covert flv format video to h264 codec with mp4 format video, but am facing the following error:
Command I executed:
ffmpeg -i input_video.flv -c:v libx264 output.mp4
Error:
Error while opening encoder for output stream #0:1 - maybe incorrect parameters such as bit_rate, rate, width or height
Logs:
ffmpeg version 2.7.2 Copyright (c) 2000-2015 the FFmpeg developers
built with Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
configuration: --prefix=/usr/local/Cellar/ffmpeg/2.7.2 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --enable-opencl --cc=clang --host-cflags= --host-ldflags= --enable-libx264 --enable-libmp3lame --enable-libvo-aacenc --enable-libxvid --enable-vda
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
libavresample 2. 1. 0 / 2. 1. 0
libswscale 3. 1.101 / 3. 1.101
libswresample 1. 2.100 / 1. 2.100
libpostproc 53. 3.100 / 53. 3.100
Input #0, flv, from 'input.flv':
Metadata:
encoder : Lavf53.24.2
Duration: 00:00:53.32, start: 0.000000, bitrate: 787 kb/s
Stream #0:0: Video: flv1, yuv420p, 320x240, 500 kb/s, 25 fps, 25 tbr, 1k tbn, 1k tbc
Stream #0:1: Audio: aac (LC), 48000 Hz, 5.1, fltp, 384 kb/s
[libx264 # 0x7ff885000c00] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 AVX2 LZCNT BMI2
[libx264 # 0x7ff885000c00] profile High, level 1.3
[libx264 # 0x7ff885000c00] 264 - core 144 r2533 c8a773e - H.264/MPEG-4 AVC codec - Copyleft 2003-2015 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
[libvo_aacenc # 0x7ff885001e00] Unable to set encoding parameters
Output #0, mp4, to 'output1.mp4':
Metadata:
encoder : Lavf53.24.2
Stream #0:0: Video: h264 (libx264), yuv420p, 320x240, q=-1--1, 25 fps, 25 tbn, 25 tbc
Metadata:
encoder : Lavc56.41.100 libx264
Stream #0:1: Audio: aac, 0 channels, 128 kb/s
Metadata:
encoder : Lavc56.41.100 libvo_aacenc
Stream mapping:
Stream #0:0 -> #0:0 (flv1 (flv) -> h264 (libx264))
Stream #0:1 -> #0:1 (aac (native) -> aac (libvo_aacenc))
Error while opening encoder for output stream #0:1 - maybe incorrect parameters such as bit_rate, rate, width or height
What could be the reason for this ?

Answer 1

Update: Based on the console output now included in the Q, the error has nothing to do with the video, but I'll leave my earlier answer below in case anyone finding this question does get the same error from the video encoder.
The error is being reported by the 3rd party AAC encoder, which is now obsolete, and only supports two channels.
There are three ways to tackle this:
#1 Copy audio stream over
ffmpeg -i input.flv -c:a copy output.mp4
#2 Upgrade to recent FFmpeg and run original command in the question
#3 Re-encode using current binary and old encoder (least recommended)
ffmpeg -i input.flv -ac 2 -b:a 128k -strict -2 output.mp4
Older answer:
Most common reason for this error is that the dimensions aren't a multiple of two, and FFmpeg is outputting using pixel encoding YUV420P which requires them to be.
Try
ffmpeg -i input.flv -vf "scale=2*trunc(iw/2):-2" output.mp4
The scale filter makes both the dimensions even.