The funny thing is that said .flv files could be played with VLC without any issue at the time I acquired them. I downloaded a bunch of cartoon episodes in this file format back in 2010 (?) when once-click-hosters like Megavideo were a big thing then. I was able to play them with the then current version of VLC without any problems.
Since then there were several updates with VLC and some time along the way it suddenly didn’t work that good anymore. I might add that this file format is not very common today (it was, when Adobe Flash was still around), so today there might be no incentive to maintain any old codecs for these type of files any longer.
When it gets worse with dwindling playback compatibility I probably have to acquire these files with a more recent file format (e.g. .mp4) in the future.