Trying to open a LibreOffice spreadsheet on an NFS-mounted directory within my home network. LibreOffice comes up with "locked for editing by unknown user".
Many posts will tell you to look for a hidden lock file in the same directory: I had none. In fact there were no files anywhere of any consequence with the names *lck* or *lock*.
I found a comment on an Ubuntu LibreOffice bug 1751005 which corresponded with some AppArmor messages I found in the local client log, and took the requisite action: I still had the problem. I'm now running LibreOffice 6 so that action may have been unnecessary, actually.
I tailed the server log and attempted the failing access: instantly there came a lockd: cannot monitor myclient message.
Finally I found the problem: rpc-statd.service was not running on the Ubuntu 18.04 server which was providing the NFS mounts. I enabled the service and all is now well.