I have the same issue. Since 2023.6 MJPEG cameras don’t show up in my lovelace dashboards.
Randomly it will show some of my 8 cameras, the others show a blank image (grey picture like in the first post). In the logs, I have many errors like :
ERROR (MainThread) [homeassistant.components.mjpeg] Error getting new camera image from cam8
But I can open my mjpeg links in chrome or vlc without issues. I can ping my nvr from my docker and host without drops. The issue seems to only come from home assistant.