One of my 3 outdoor cameras not workimg

All 3 outdoor cameras were working OK, then one stopped recording videos.
If you press the ‘photo’ or ‘video’ button the motion sensor/infra red? dome flashes red 6 times, but no photo or video is recorded.
A message says ‘thumbnail failed’.
All seems OK in the settings section, with battery showing ‘OK’ and good WiFi and Hub signals.
Could you tell me what the flashing red means, and what to do to correct the problem please.
Thanks, Gerry

Most likely failing batteries, see the following link:

App notification cannot be trusted.