XnView Software

FLV video files are an old container format for flash videos or other movies. Flash player is no longer supported by chrome, firefox and other browsers. Every popular browser removed this standard. The flv files contain a h263/h264 or flash video stream and mp3/aac sound, which you can convert to ...

I could see that XnViewMP uses the ffmpeg shared libraries for different jobs (e.g. video playback, etc.).
Maybe you can update the files to the lastest version 4.4.1, because the current files are from an earlier ffmpeg (4.x) release of 2020.

I replaced them in the XnViewMP directory by myself ...