Movian

O, I figured out that using a USB-Flashdrive everything works fine. Seems that DLNA can't handle srt files.

Some DLNA servers can handle SRT (i know it works with minidlna) but it's not very standard and the file must have the exact same name as the movie file (except the file ending ofcourse)

DLNA itself doesn't support separate subtitles (sadly), the only way to have subtitles with DLNA is if the server supports it, but you have to select it before you start the movie and the 'subtitle' button on your remote will not work.. It's one of the biggest shortcommings of DLNA.
I don't know if it's even possible to register the subtitle button on the DLNA server side (so if you press the button the code/signal is also sent to the server), there is hardly any documentation available on the internet about the whole protocol (at least I haven't found it).

I have a WD MyBook Live NAS that runs Twonky Version 5. It should work with later versions, but as WD doesn't want to upgrade Twonky and I am not willing to experiment with manual upgrading I will stay without subtitles. But for me it's not that big issue, as I can also play the movie from a USB drive.

try ps3mediaserver

plex is great too

Tried plex media server, but ditched it right aways, it worked even worse than the latest universal media server. Most video's didn't even play and there isn't an option just to browse folders, you have to first create a 'database' of all the movies/music on your machine which takes a while..

Would be great though if separate subtitles could be served as some servers do seem to support it (after I did some investigation) but they also needed special clients, which ofcourse the PS3 DNLA client doesn't do, but I don't know about showtime (especially in combination with PS3MS or UMS, but the function could be added to the servers).