@catcamstar Your post encouraged me to spend some more time on this issue. Tried lots of things (including re-installing the app), but finally I found the solution. If anybody runs into the same issue, here's how it works.
In the NVR is a setting 'Maximum connection' (Network -> Connections). The value was 5. I've changed it to 128, the max. (Probably a much lower setting will work as well).
I don't know how the updated iDMSS app works, but it seems that it logs into the NVR multiple times, probably for every cam (and every time) that is displayed. This would explain why I could see cams initially when I started the app, but when scrolling forward and back to other cams, it failed. Restarting the app triggered the "Maximum connections reached" error. The old iDMSS app apparently worked differently, because it used to work great.
Playback seems to work too, although I'm still confused what to choose. I use IVS on all my cams and I would assume that this would be shown under "Motion detect", but it always tells me there's nothing recorded. Under " Normal", I can see markers when IVS was activated, so this kinda works.