I'd guess the Oppo is responding too slow for us.
There are a few tests you could try and it may come down to doing a wireshark trace for debugging.
First, to eliminate variables, make sure there is no format conversion happening. Go in to your MC DLNA server settings
Options->Media Network-> add or configure DLNA servers... {choose the server}
Audio->Mode
set to "Original"
Play a flac file. See if it keeps playing or not.
Are you pulling the files FROM the Oppo or pushing TO the Oppo from MC?
If you are pushing, you can try this.
In MC's Playing Now find the Oppo zone, right-click on it and at the bottom of the list you'll see DLNA Controller Options.
Check the one that says "Disable SetNext support"
Try to play a flac file again.
If it still fails, Check the second DLNA Controller option, "Disable Transport Events"
and try playing a flac file again.
If it still fails you'll need to do a wireshark capture.