Normally, you'd get this error (ECHILD) if you wait for a child, but there is no child, e.g.

$ perl -e 'die $! if wait == -1'
No child processes at -e line 1.
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In other words, I'm not sure if this is (directly) related to some resource limit at all... (though, of course, it might be a follow-up error of some code doing a wait for a child that never had been created, due to a resource limit like memory, or max children per user).