I got this error spaming the console. NULL GEngine->GetSubtitleFont() - subtitles not rendering!
After some searching google found nothing, so then we searched around the Unreal Engine source code.
Tured out that our DefaultEngine.ini had two definitions for the various fonts and that was confusing it.
(Maybe a bad merge by Git?)
I hand-edited DefaultEngine so only unique lines for fons were present. That solved it.
Chears!