Thank you both for the help.
It is a first installation on this particular Debian machine. I have run previous Processing versions on other Debian sid systems with no problems.
The error is triggered literally simply by unpacking the .tgz archive and launching ./processing. I’m not running the install.sh script because I don’t need shortcuts or file associations.
The java shipped with Processing is java 1.8.0_181, but I don’t know how revealing that is given that it is shipped with the software.