This comes from Main HD is full. Home Assistant on Docker installation
After deleting /var/log/kern.log (which was about 8GB) I started monitoring it (tail) and nothing was written to it.
All I can see now is the following output to stdout (its a laptop):
EXT4-fs error (device sdb1): __ext4_find_entry:1531: inode #34340865: comm backup cleanup: reading directory lblock 0