Steps to reproduce:

1. Download Caddy: https://caddyserver.com/
   (I tested with v2.4.5.)
2. Run an HTTP server for testing:
   caddy.exe file-server
3. Build and run the attached HTTP client project: 20210927-1018-QNetworkAccessManagerTest.zip.
4. After around 4,000 consecutive GET requests (about 15 minutes), the application crashes with a read access violation.

This bug could possibly be a duplicate of QTBUG-38309, however:

• The other bug specifically says it does not happen on Windows whereas this one does.
• The other bug points suspicion at OpenSSL whereas this bug uses HTTP.

Having said that, I tried the same work-around mentioned in QTBUG-38309, changing:

reply->deleteLater();

to

QTimer::singleShot(1000, reply, &QObject::deleteLater);

 and so far (over 13,000 requests / 44 minutes) it has not crashed.