This has started since 7.2 ( 7.2-4100-04). I had 2 Rokus ( all 4200) update and one still on 7.1 so i could compare where the firmware was the only difference.
A movie playing from a Synology NAS will spontaneously stop, as if the back key were pressed. This happens 10 - 20 minutes in. If you restart the movie from the beginning, it will exit at the same spot every time. Trying to skip ahead will exit immediately. This is with my own (SDK based) media player but The Roku Media Player shows the same behavior. It throws an error about "corrupt or wrong format" etc.
Of course the same exact same file will play fine if sent to the AppleTV or Mac.
There's nothing to fix, as nothing errors in the console, it's seemingly random, and the files themselves are OK.