No Video without OSD

Hi together,
I’m trying to transmit a clean video signal with OpenHD. Everything works with DISPLAY_OSD=Y, but with DISPLAY_OSD=N, I only get a black screen on the receiver.
All other settings are default. Everything else works out-of-the-box!

TX is RPI0 with RPi-Cam. RX is RPi4, both with AC56. I set it to “N” on both RPis. Maybe I’m missing anything else?
Thanks and regards
Andreas