OSMC with Kodi 19.1 Playback of large video files stuck

Ok, was an overlook on my side. The MTU fix is only applicable on the 64Bit platforms.

I suggest to try SMB3 and see if that helps. Maybe the issue is something totally different. Things you could test:

  1. Test network on the Pi3 with IPerf3
  2. Try to play the file locally from USB

Maybe something changed in the network, we suggest to check your network with iperf3. Please read this howto