Another device on the network is using your computer's IP address...

Have you tried power cycling your router (or modem/router)? Or checked your router's logs and configuration?

The macOS system logs are pretty much useless for troubleshooting these days. Normally you can use the Console app to locate & view the various system logs, but Apple thinks so highly of these logs that many of the logs no longer appear within the Console app. Good luck trying to find anything useful in them. The logs are located at these locations:

• /Library/Logs/DiasgnosticReports
• /private/var/log (I also see /private/var/logs on some systems which may contain a few entries --notice the "s" for some app logs as Apple cannot even be consistent in their naming of the folder -- but not sure what is using it as I ignore it)
• ~/Library/Logs

This looks like you have two DHCP servers in your network. A DHCP server hands out IP addresses to other devices. When you have more than one such DHCP server a device requesting an address is served by the one with the faster reaction. This can (and often does) result in an address collisions. As to "everything is stable during the night". Well, a computer or other dynamically configured device (like an access point or a phone) is trying to renew its address on a more or less regular base. This is no problem if it is always the same DHCP server answering, but sometimes the other one is a micro second faster - and boom, address collision. Maybe the new modem is configured to act as a DHCP server but you already have something else setup for that purpose. Check the configuration of your modem and other network devices. You must have only one DHCP server in your network otherwise you will have exactly the issues you see.

Happy hunting!

There's a lot more to this, and it will take some troubleshooting to identify.

First off, every device on your local network needs a unique IP address.

There are two common ways devices can get an IP address:

Firstly, devices can be configured manually, by giving each device all the details it needs to get online (IP address, subnet mask, router address, etc.).

Secondly, devices can query a DHCP server to get those details, and the DHCP server keeps track of which device has which IP address, so it knows what addresses are available vs. in use.

Most people use DHCP because it's easy (most people don't understand those manual terms anyway); those details are repetitive; tracking which device has which IP address; and change is hard (having to walk through every device on the network to make changes).

If *everything* on the network is configured to use DHCP, and there's only one DHCP server, the DHCP server knows which IP addresses have been issued, and this problem should not occur.

So this kind of problem occurs because of one of two conditions.

a) one device on the network is configured with a static IP address (in this case, 192.168.254.12). It doesn't care (or know) that a DHCP server is trying to hand out that IP address, and the DHCP server doesn't know about the statically-configured device, and so the conflict message appears.

b) there are two DHCP servers on the network that both think they are responsible for the 192.168.254.12 address. Neither of them know about the other server, so the second server doesn't know the first server has already handed out the address.

Note: there is NO typical way to control which DHCP server responds to a request on the network - when a device connects to the network, it sends out a DHCP request; any DHCP server on the network sends a response, and the first response the device sees is what it uses. For this reason, running multiple DHCP servers on a single network is not recommended, unless you're a senior network administrator and really know what you're doing.

So, in order to resolve this, you're going to need to identify the other machine on the network that's triggering the issue - either a duplicate server, or a rogue client.

Timing may be your best friend - try to identify what other devices were just turned on, or if someone just walked into the office with their laptop, etc.

If you have access to the DHCP server, its logs should indicate what IP address was handed out to each device, so that might help identify the culprit.

You *might* find some useful data in the OS logs, but that won't be more than a MAC address (although this might be useful in at least identifying the type of device you're trying to track down).

Since the problem is on WiFi, it could be some rogue user connecting to your WiFi network, so check your WiFi settings, maybe changing the WiFi password, or checking the WiFI logs to see what devices are connected (the ease of doing this will depend a lot on your WiFi router)

This happens when one computer is offline, and another joins. DHCP assigns the new computer an address and when the offline computer joins up it just trys to use the same address. This is benign.