Plugin breaks after first setup – no more configuration possible

Hello,
thank you for your awesome work on this plugin!
I just set up a new WP-instance and unfortunately Videopack breaks every time I configure it for the first time.
I just receive the message on the bottom of the site, that a serious error occurred. After that the account-tab isn’t there anymore and one can’t configure anything else.
Deactivating and Reactivating also doesn’t help.
Only when I delete the plugin and reinstall it I am able to enter the setup again until the first save – then the above error occurred again.
A look on the WP_debug.log (attached below) shows some TypeError-error, but unfortunately I don’t speak PHP and can barely read it.
Could you please point me the direction for further troubleshooting?

WP-instance-specs:
bare-metal-server
Ubuntu 20.04
PHP 8.0.5
WP 5.7.1

Output of suggested FFmpeg “Test-Command”

ffmpeg version 4.2.4-1ubuntu0.1 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-10ubuntu2)
  configuration: --prefix=/usr --extra-version=1ubuntu0.1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-nvenc --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/var/www/ambientvideo/wp-content/plugins/video-embed-thumbnail-generator/images/sample-video-h264.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf55.33.100
  Duration: 00:00:01.02, start: 0.000000, bitrate: 123 kb/s
    Stream #0:0(eng): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 109 kb/s, 23.98 fps, 23.98 tbr, 11988 tbn, 47.95 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 2 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> mjpeg (native))
Press [q] to stop, [?] for help
[swscaler @ 0x563535a44c00] deprecated pixel format used, make sure you did set range correctly
Output #0, mjpeg, to '/var/www/ambientvideo/wp-content/uploads/media_temp/ffmpeg_exists_test.jpg':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.29.100
    Stream #0:0(eng): Video: mjpeg, yuvj420p(pc), 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 23.98 fps, 23.98 tbn, 23.98 tbc (default)
    Metadata:
      handler_name    : VideoHandler
      encoder         : Lavc58.54.100 mjpeg
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
frame=    1 fps=0.0 q=7.6 Lsize=      22kB time=00:00:00.04 bitrate=4314.2kbits/s speed=1.17x
video:22kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.000000%

WP-Debug-Log:

[03-May-2021 23:23:57 UTC] PHP Fatal error:  Uncaught TypeError: array_key_exists(): Argument #2 ($array) must be of type array, bool given in /var/www/ambientvideo/wp-content/plugins/video-embed-thumbnail-generator/video-embed-thumbnail-generator.php:5175
Stack trace:
#0 /var/www/ambientvideo/wp-admin/includes/template.php(1737): kgvid_encode_formats_callback()
#1 /var/www/ambientvideo/wp-content/plugins/video-embed-thumbnail-generator/video-embed-thumbnail-generator.php(4703): do_settings_fields()
#2 /var/www/ambientvideo/wp-content/plugins/video-embed-thumbnail-generator/video-embed-thumbnail-generator.php(4584): kgvid_do_settings_sections()
#3 /var/www/ambientvideo/wp-includes/class-wp-hook.php(292): kgvid_settings_page()
#4 /var/www/ambientvideo/wp-includes/class-wp-hook.php(316): WP_Hook->apply_filters()
#5 /var/www/ambientvideo/wp-includes/plugin.php(484): WP_Hook->do_action()
#6 /var/www/ambientvideo/wp-admin/admin.php(259): do_action()
#7 /var/www/ambientvideo/wp-admin/options-general.php(10): require_once('...')
#8 {main}
  thrown in /var/www/ambientvideo/wp-content/plugins/video-embed-thumbnail-generator/video-embed-thumbnail-generator.php on line 5175