I have tried everything with a lot of combinations on a brand new Raspberry PI 4 B. Nothing worked.
I installed older versions of Raspberry PI OS, tried various suggestions on the forum here (rPi4 install not working).
Searched on Github: Screenly bootscreen shows, then blackscreen Raspberry Pi 4B · Issue #1580 · Screenly/screenly-ose · GitHub
After days of testing and many many installs I gave up.