Serviio 1.4 - Zyxel nsa320 no subtitles

I bought a Zyxel nsa320 with twonky pre-installed. But I had always a black screen when playing any file. So I've installed Serviio 1.4 on the nas. Al my files are playing now without any problem, except that I cannot see any subtitle.

So I'm running Serviio 1.4 - java version 1.7.0_51 and ffmpeg version 2.0.1 on my Zyxel nsa320 playing on my Sony KDL-HX850. When I play the files with Serviio 1.4 installed on my laptop I have no problem with subtitles.

On the Serviio console from the nsa320 I have enabled subtitles (delivery/subtitles). I've tried all the different Sony Bravia profiles. I've decoded the srt files with notepad++ to UTF-8 format. The srt file has the same name as the movie file. But still no subtitles.

I've compared the settings of Serviio on the laptop with the settings on the nsa320 Serviio console, and they are the same.

I've no idea why the subtitles works on my laptop and not of my nas.

Anyone any ideas?

Thanks