I've found what was eating the RAM, had nothing to do with the upgrade... maybe the behavior is different or available RAM lower which is causing the reboot.
Sorry for this false alarm
I've upgraded my Proxmox server from v8 to v9 a few days ago.
It rebooted several times since the upgrade and it looks like it is caused by some RAM exhaust problem.
Total RAM is 256GB. All my CT+VM are using roughly 64GB of RAM and their...