I’m having the same issue as @FahimF. I upgraded my Ubuntu Linux Server version from 1.16.3 to 1.16.4 and got the problem. It persisted to the 1.16.5 upgrade as well. Downgrading the server to 1.16.3 eliminated the issue. My clients are also mainly Roku. Source encoding has been varied: .avi, .mkv, and .mp4.
I started a thread here: Transcoder issues since 1.16.4, which includes logs. @Deme74 suggested this thread may be the same issue and I think that may be the case. However, I never would have found this thread since I believe the issue is on the server, not the client (or not on all clients, anyway).
