Leo, I had a similar problem with a recording and Support gave me a page of information to investigate.
My problem wasn't the black void, just an audio void.
I found out that:
BBB recordings start with the entry of the first person. If that person enters early, you have to scroll forward to find the actual start of the lecture (beginning of audio). For Martin's keynote, I had to scroll 50% forward before the lecture began. Unless you scroll forward, you'll not find it.
Just in case you get a viewable screen but still a problem....