I tried to find the problem and strace'd the proxmox backup task's pid. (it was 26074). it was looping like this;
stat("/etc/pve/nodes/node02/qemu-server/99999.conf", {st_dev=makedev(0, 22), st_ino=1101, st_mode=S_IFREG|0640, st_nlink=1, st_uid=0, st_gid=33, st_blksize=4096, st_blocks=1...