Well, you might not like what I found. That sample worked fine for me using my own channel and own server. Pardon me for asking a stupid question, but you did unzip the downloaded file, right? Open your SRT file in any text editor and it should be readable. If it's full of garbage characters (except for the first 4 characters in the file which may be the UTF-8 BOM) then something's wrong with the file.
I believe Plex uses the firmware's support for SRTs, but I don't use it so I can't say for sure. Have you tried the Roku Media Player channel? It should find your Plex server if it's setup to be a DLNA server and as long as the video and SRT file have the same name it should pick it up. Once again, I don't use RMP or Plex, so I can't comment on them or test them.
On the bright side, there's nothing wrong with the downloaded file and the Roku shouldn't have any problem with it, but we're no closer to figuring out what the problem is.
-JT
Roku Community Streaming Expert
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