Thx for pointing out "journalctl"
found it:
Oct 28 05:21:27 pve kernel: oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=qemu.slice,mems_allowed=0,global_oom,task_memcg=/qemu.slice/104.scope,task=kvm,pid=3888245,uid=0
Oct 28 05:21:27 pve kernel: Out of memory: Killed process...