[RESOLVED]
I don't know whether I said this somewhere else, the cause was indeed a buggy Ghostscript. This is what our FreeBSD specialist found out:
Replacing ghostscript9-base 9.06_14 with ghostscript9-agpl-base 9.52 solved the problem.
So a bug from A.D. 2011 resolved in A.D. 2014 was sleeping in our FreeBSD in A.D. 2020 and caused some havoc. It created 900 GB (no typo) big log files, the 500 k files in filedir/lock.
Possibly this is at least partly responsible for exhausted memory - although 256 GB physical memory was available. Found log files with records like
PHP Fatal error: Allowed memory size of 536870912 bytes exhausted \
(tried to allocate 2323399 bytes) in /usr/local/www/.../
running for months.
Hope to lay the matter at rest.