Error opening MKV - missing codecs

[soap2]

When I try to open a MKV x264 file it will give me an error that the file cannot be played because codecs are missing. I have LAV filters installed on system and file plays fine in all other players.
I even tried reinstalling codecs using the K-Lite pack the player error linked me to, but even with KLite installed, still I get the same error and file won't play in GFP due to "missing codecs". What am I doing wrong?