Did you find something new about this issue?
In between, I've realized that it's the same issue here in this forum. This youtube video here, has the same black bars, when I view it here on my imac:
So it's probably a generall issue, that could be solved in a softare update?
Thanks for your reply
PS: The forum link I send you ( https://www.freiklavierspielen.com/community/ ) is no longer public available, since it's a forum only for paid members and it's going live today.