The fact that the problem is intermittent leads me to believe that the apache server was not stopped and restarted after the upgrade. This means that some of the child processes are still operating using software that was loaded when they started (before the new modules such as PGcore were added). A child process that was created more recently won't have this problem.
Let us know whether restarting the apache server corrects the problem.
-- Mike
WeBWorK Main Forum
Problems with Webwork "PGCore" after upgrading PG to head version
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