Does the flac file play with ffplay (from ffmpeg) or mpv? Is it any/every flac file or only ones you've recorded? If you don't have any other than the ones you've recorded, I'd recommend downloading something from LMA and trying those files in vlc, ffplay, mpv.
https://github.com/BtbN/FFmpeg-Builds/releases
https://github.com/shinchiro/mpv-winbuild-cmake/releases
I've got a fresh install of Windows so not sure I want to add a bunch of new software right now, but they're not playing in Windows Media Player and VLC.
Smaller flac's were playing just fine, but every large one (usually 2 gigs due to splits on the recorder) but a few files that were around 1.5 gigs also had the same problem. I had recordings from my F3, F6, Tascam DR-05, and H1essential all have the same problem.
All I know is that out of nowhere, around late August, I followed my exact same routine of recording a show, copying the files onto my desktop PC, writing the info file etc and flac'ing the files. I usually like to test the flac's (by playing them) to make sure everything went well and it will eventually be safe to delete off my recorder. Well sure enough the flac's weren't playing, and Windows Media Player gave the error message "We can't open <file name>. It's encoded in a format that isn't supported." I tried the files in VLC and it simply wouldn't play the files. When I coverted the files back to wav (using freac) they played fine.
As mentioned before, all my old recordings, 2 gig flac files included, are playing just fine.
It's gotta be Windows and their new problems with large files, as documented on the web and by the post by Gone Fishin in the Post-Processing section. It happened on my Windows 10 PC right when that new Windows patch or update caused problems for others (and yes an update was installed before it happened). I tried rolling back to a previous edition and it didn't help. That computer was pretty old and starting to give me different problems so I jumped on a chance to get a new PC from a friend. Was hoping a fresh install of everything would solve all those problems, but here we are. Just wondering if I'm the only taper experiencing this.
Someone suggested above that I try DB Poweramp, and I went to their site but it looks like $48 bucks. I suppose I just need to try the trial version and maybe isolate freac as the problem. Curious that no other tapers are reporting this problem however. Like I said, once I convert back to wav the files are fine, or at least they play fine in WMP and VLC. That little OCD in me though is hesitate to archive these "masters" when there MIGHT be a problem with the flac conversion. *sigh, stupid technology.
EDIT: might try Foobar2000, will report here.