This turned out to be a PostgreSQL database issue (corrupt database/table/index).
Search in
/var/log/postgresql/postresql-9.6-main.log
for entries like
2020-01-07 13:36:15.599 EST [15852] ERROR: MultiXactId 1583362 has not been created yet -- apparent wraparound
2020-01-07 13:36:15.599 EST [15852] CONTEXT: automatic analyze of table "db_nextcloud.public.oc_jobs"
My solution was to use a new database, reinstall from scratch and use
root@wolke7:/var/www/nextcloud# sudo -u www-data php /var/www/nextcloud/occ files:scan --all
to populate the database again.
Thanks, -MN