@mat200 the logfiles are not showing unexpected reboots. Nope, it's not connected to a UPS but with the original power adapter.
Wednesday I manually rebooted the NVR 3-4 times and kept the DHCP address.
Let's hope that the changes of my last post have a positive effect.
@bigredfish , all (2) camera's are directly connected using PoE to the NVR.
One of the RJ-45 connectors has been replaced a couple of months ago due to faulty connectivity. This is since then stable and keeps recording also when the NVR's IP-address is being lost. So the recording settings are intact and the Playback feature works perfectly.