Strange! The configuration looks fine from here. If you find more evidence that would help understand the problem, such as messages in your verbose log, we may be able to help.
I checked the log for the import with --verbose the convert portion of the log just says this for each file in the album. The ffmpeg command definitely works as i tested on a single file to see if it worked, so I’m not sure why its saying it failed
convert: Encoding /mnt/storage/Downloads/The Fall - Extricate (2002) CD FLAC/01 - Sing! Harpy.flac
convert: Encoding /mnt/storage/Downloads/The Fall - Extricate (2002) CD FLAC/01 - Sing! Harpy.flac failed.
EDIT:
I didnt realize verbose had different levels, here is the output with -vvv
ffmpeg version 3.2.10-1~deb9u1 Copyright (c) 2000-2018 the FFmpeg developers
built with gcc 6.3.0 (Debian 6.3.0-18) 20170516
configuration: --prefix=/usr --extra-version='1~deb9u1' --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --disable-stripping --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libebur128 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmp3lame --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-omx --enable-openal --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libopencv --enable-libx264 --enable-shared
libavutil 55. 34.101 / 55. 34.101
libavcodec 57. 64.101 / 57. 64.101
libavformat 57. 56.101 / 57. 56.101
libavdevice 57. 1.100 / 57. 1.100
libavfilter 6. 65.100 / 6. 65.100
libavresample 3. 1. 0 / 3. 1. 0
libswscale 4. 2.100 / 4. 2.100
libswresample 2. 3.100 / 2. 3.100
libpostproc 54. 1.100 / 54. 1.100
Input #0, flac, from '/mnt/storage/Downloads/The Fall - Extricate (2002) CD FLAC/03 - Bill Is Dead.flac':
Metadata:
ARTIST : The Fall
TITLE : Bill Is Dead
ALBUM : Extricate
DATE : 2002
track : 03
album_artist : The Fall
disc : 1
TOTALDISCS : 1
TOTALTRACKS : 14
Duration: 00:04:32.93, start: 0.000000, bitrate: 898 kb/s
Stream #0:0: Audio: flac, 44100 Hz, stereo, s16
File '/tmp/tmplvnracjx.mp3' already exists. Overwrite ? [y/N] Not overwriting - exiting