I am finally able to report some progress on understanding the problem, and provide a workaround.
This bug occurs when the system compiled with openjdk8 is run under the openjdk11 runtime. Inconsistently, the abcl-1.7.0 binaries were compiled with openjdk11 while the abcl-1.7.1 binaries were compiled with openjdk8.
In openjdk11, the java.lang.UNIXProcess class has been replaced with java.lang.ProcessImpl, so our code needs to conditionally find the pid of a given process based on the platform that the ABCL finds itself running upon.
As a workaround until we fix and release the next version of ABCL, one may get sys:run-program running as long as one doesn't try to get the pid via first running the following code which is sufficient to load sys:run-program:
(ignore-errors (sys:run-program "true" nil))
An easy fix for users of the abcl-1.7.1 binaries under openjdk11 would be to place such code in file:~/.abclrc.