Apache appears to stop processing requests due to high number of "scoreboard is full, not at MaxRequestWorkers" errors (AH00485) within log file. No recent changes to config files or setup. This occurred in high volume production web server leading to an outage. Only response I can find appears to be rolling back Apache version (not ideal given prod environment etc). There have been suggestions of config file changes although I cannot find a great deal of information on these recommendations. Symptoms appear similar to bug #53555.
Marking this report as duplicate of 53555, that has been fixed and released in 2.4.25. Please test the new httpd version and if you still see the bug, re-open this task so we'll follow up again. Thanks! *** This bug has been marked as a duplicate of bug 53555 ***