@mmavrof: We’ve tested OS 2.3 DC for a long time on FW 2.1.9 and didn’t observe the issues you reported. It has been connected to our test network through a WiFi adapter for days and there was no reboots.
Reboot reason 99 just means the controller experienced a complete reboot sequence, it didn’t mean you manually switched it on/off, it means something else is causing it to reboot, could be a power adapter issue, or something else.
The only reason I could think of, that would explain why 2.1.7 works fine but not 2.1.9 in your case is that 2.1.9 uses the UIPEthernet library, which is a popular open-source library for managing Ethernet connections. This might be causing issue on your particular network, but as I said, we have a test OS 2.3 DC on our network for days without any issue.
One thing to check is: does your controller exhibit the issues reported under factory default settings, or after you have imported your previous configurations? If it’s the latter, it could be that some configurations are causing the issue, which we are not aware of.