It seems that the multiple jobs parameter is the cause. If I specify "-j 1", the command works without problems. If I specify "-j 2" or higher, I get the above error after a few seconds.
Postgres is running on a Mac Pro 12-core machine, so it has plenty of resources at its disposal. The config file is a copy of the Postgres 11 configuration, which has no problem with multiple jobs.
Furthermore, the pg_dump command seems to have the same issue as well. The following command: