Just in case this helps someone:
One of our customers encountered the same problem today.
We had the impression that application sharing sometimes worked with small application windows, but worked rarer with increasing size of the application window.
We observed that the problem only occurred with one, even the slower one of the two internet connections.
Those connections are quite equal (apart from the speed), but are using different DNS servers.
When the internet traffic was routed through the first connection which was using the cloudflare DNS 1.1.1.1 - other people only saw black screens when sharing the screen or an application.
When the internet traffic was routed through the second connection, screen sharing worked instantly.
So we changed the DNS server of the first connection for test purposes - and after the line was disconnected an connected again, screen sharing worked like a charm via both connections.
Interestingly it still worked after changing the DNS back to 1.1.1.1 - so it's certainly possible that only the reconnect solved problem and the cloudflare DNS not to blame.