Few days ago I got the "Watchdog detected problem." in my log and the camera hanged with a freezed preview picture.
I use a RPi 4B with 2G memory and the original power supply with stiff power cable. Mains power is very stable where I live so I don't think it's a brown-out. The cam is mounted on the wall near a hall way and has worked fine several months so I started to search here.
From reading btidey's reply to antondubek on page 174, he says that;
"The watchdog is looking for the cam.jpg file in the /dev/shm/mjpeg folder to be regularly updated. This is the preview file which forms the stream. If this does not update for more than the watchdog period then the main process kills itself and the watchdog process starts a separate copy.
As your watchdog is kicking in before it gets going then this would indicate that the camera process is not producing anything right from the start."
So something goes wrong in the camera and this happened to me shortly after midnight, so I had no hard recording at the time.
If you look at the camera, it's a tiny pcb with a tiny flat cable and connectors to a small pcb and a 15 cm "long" 15 pin ribbon cable to the RPi and "nothing" what we could call "ground" in RF sense.
We have all read about checking and re-checking all connections AND power supply, so this has been done several times.
However the ground connection to the camera is not very well defined and I see no obvious EMC filtering. Few capacitors on the camera board and eight on the camera itself could be EMI related filtering. For normal use I think this is good enough, but if you bring your dear phone nearby you might end up with a surprise.
I never carry my phone with me, but my kids do 24/7 for some reason. Even if you think you don't use your phone, the master spy himself does to check what you are up to, just as he is here on this forum to learn few things
So, if you have problem with your camera and checked everything else you can imagine, check also this:
Leave your mobile phone at least two meters or more away.
If that is not doable, place the RPi with camera inside a metallic box with a hole for the lens and see if it gets better. I have used those cheap one time use food boxes that you can easily bend to shape and put in the freezer or oven. Just isolate it inside to prevent short circuits and avoid hot places as the air cooling gets worse. The RPi 3B+ consumes some 3-4W during operation and 4B even more, so it may not like it without a fan. Warranty void as usual.
Other EMC tricks is to add ferrite clamps on cables including the power and camera cable.