Hmm. No telling what differences there are between the ps3 media server and serviio. I guess it is a serviio issue, but still it's odd that it doesn't talk properly with the S790 while another DLNA server does.
From Sony's esupport for the S790:
That Sony support message may be about having an .srt file next to the video file. What works for me is adding any external subs into the mkv file with MKVtoolnix (which uses mkvmerge).
(http://www.bunkus.org/videotools/mkvtoolnix/). You can also use this app to add chapters, fix RW/FF issues and remove other data tracks added by some encoders that may cause playback issues.
I think I'm narrowing my other problem down though. I've found that some properly muxed mkv files don't FF or Rew and when that is the case it is prone to the playback stopping. Trying to replay, removing/reinserting the USB stick doesn't fix it. But if I put it in my PC and check the files, when I put it back in the S790 the FF/rew function work.
I'm wondering if my problem is something stupid like me not 'safely' removing the usb stick..
Any FF/RW problems I've had have been solved by re-muxing the mkv with MKVtoolnix with the following command line options added which you can also add with the GUI.
--engage no_cue_duration --engage no_cue_relative_position
Then I can FF/RW without it stopping the video.