This network crash issue (incoming connections are not working - sometimes mainUI is still ok - see above) is really annoying and I have followed some other threads about similar issues.
There are a few rough ideas which might be related to this issue:
- Corrupt file system: Originally it happend on my SD-card, but it’s happening now also using the SSD
- Low voltage: Slow zwave (USB) and SSD (USB) cause the SSD to disconnect (?)
(network disconnect was also mentioned for RPi1-3, but the RPi 4 Ethernet is not connected via USB bus). I am using the official RPi4 Power supply. Furthermore the red LED is solid ON.
side note: The SSD was connected as backup, even when running from sd-card. - One thread mentioned the zwave stick beeing not connected properly to a RPi4 (via USB2), but this does not seem to be the case on my setup (it connects properly to /dev/ttyACM0).
The next step is to start from scratch (what I already did a few times) and just hope.
I bought a brand new SSD and will go for the ZRAM option with UPS.
Usually I am an optimistic guy, but this keeps me awake at night 
Any more ideas are still greatly appreciated!
Thanks for all your help guys!
An important reference is this by the way (thank you for this @mstormi)