Backup from HAOS Raspberry 4, Restored on HAOS Raspberry 3B - Hangs / Looses Connection

My Raspberry Pi 4 (4GB) is my Main HA Environment, running on HassOS.
I want to prepare my second Raspberry Pi 3 (1GB) as the Fallback-Machine, in Case of Hard/Software-Failures on the Main Machine.
I flashed the MicroSD-Card with HomeAssistant OS, this Time of Course for RPi3 64bit, and it is booting up successfully. It runs as well after Restoring the latest Backup. But unfortunatly, after 1-2 Hours, it is becoming less and less stable, looses Connection, til there is literally nothing i can do except pulling the Plug.

The last Time when i was able to check System, before it became unstable/unreachable, it gave me: CoreCPU: 2%, CoreRAM 20%, SupervisorCPU 0%, SupervisorRAM 4%. So to me, it doesnt look like a Problem with Ressources.
How can i find out whats the Problem? I’m barely used to Shell/SSH. Happy when i manage to follow some Step-by-Step Tutorials :smiley: Probably kinda Log, i let it run til it hangs again, and then look for something increasing til its 100%? :crazy_face:

Nobody?
Is there a RTFM i missed? Would be really grateful yet relaxed to sort out the Problems with restoring my backup to another Raspberry! :relaxed: