I know this is an old thread, but I hope this info might help save a support email. I was having a similar issue in release 5.2.4.6 x64 (04/06/20).
Problem:
When I view the clip list and timeline for a certain camera group, the clip list and timeline are both empty.
But if I choose "selected camera only" for the clip list, it correctly populates the clip list and the clips appear on the timeline for that single camera.
Solution:
Turns out that this group is made up entirely of cameras tagged "Hidden", and these cameras are excluded from the clip list & timeline unless you right-click in the clip list and turn on "include clips from excluded cameras". The confusing part here is that I have "Show hidden cameras" enabled in the main view (so I didn't even realize that they would be considered "excluded"). Also, this setting is not used when "selected camera only" for the clip list is enabled.