Interestingly, I had the same problem on another machine, but with a different lockfile:
FileExistsError: [Errno 17] File exists: '1516' -> b'/var/lib/landscape/client/sockets/monitor.sock.lock'
So I think there is some situation, possibly an upgrade from bionic to focal, which is causing lost lockfiles which then block future starts.
Interestingly, I had the same problem on another machine, but with a different lockfile:
FileExistsError: [Errno 17] File exists: '1516' -> b'/var/ lib/landscape/ client/ sockets/ monitor. sock.lock'
So I think there is some situation, possibly an upgrade from bionic to focal, which is causing lost lockfiles which then block future starts.