PI (DHCP enabled) goes down sometimes

I have phihole installed on a raspberry pi for 3 years now, never had any problems, it runned always very stable. Current version is Pi-hole v5.12, FTL v5.17 web v5.14.2

I enabled the DHCP server on the PI, and disabled it on the router. Main reason was that my internet provider's router won't allow port acces for my synology VPN, connected it all toghether by switches

My PI is normally accessable in my local network at

Since Last half year I often have the problem that my PI is suddenly not reachable anymore in my network. As it is my DHCP server, I can't connect to any of my network devices, not my NAS, not the router, so my wifi and cabled network goes totally down. I am not able to reach the reach the pi at via browser or via putty

After some trys of plugging out the power cable and rebooting, PI is reachable again, and business as usual.

How / where can I find out what the reason is of going down. (it happens ca once in the 2 weeks, very annoying.... Did I simply misconfigurate the dhcp or do I have a hardware failure or a corrupt sd card?

Please generate a debug log, upload it when prompted and post the token URL here.