This happens if the RTSP camera does not send video codec information or sends it incorrectly. The SPS and PPS parameters in the response to the RTSP DESCRIBE.

This can be fixed using ffmpeg source. But this will add a delay to the start of the stream.

streams: camera1: ffmpeg:rtsp://...

Without ffmpeg source, this requires significant tweaking of go2rtc. Specifically, analyzing the stream before starting the streaming.