the issue QTBUG-11824 seems to have been partially solved, because I still face it in case two requests are made with bad credentials (identical or not). At the second trial, I get

QNetworkAccessCache::removeEntry: trying to remove key 'ftp-connection:ftp://host:21' that is not in cache

Then I get a segfault when deleting the network access manager.
The Url used is the following ftp://user:password@host:21/data, and the signal QNetworkAccessManager::authenticationRequired is not connected.

After digging into QNetworkAccessFtpBackend, I noticed the following:
It seems like during the first request, empty credentials are cached (because of newUrl.setUserInfo(QString()); in QNetworkAccessFtpBackend::ftpDone)
At the second request, inside QNetworkAccessFtpBackend::ftpDone, it enters inside the following condition :

            QAuthenticator auth;
            authenticationRequired(&auth);

            if (!auth.isNull()) {
                // try again:
                newUrl.setUserName(auth.user());
                ftp->login(auth.user(), auth.password());
                return;
            }

where it tries to login with empty cached credential, thus the QNetwokAccessCache finally tries to remove the cache entry without credentials whereas it was cached with credentials.

As a workaround, I suggest avoiding caching empty credentials in QNetworkAccessManagerPrivate::authenticationRequired by changing

    if (allowAuthenticationReuse)
        authenticationManager->cacheCredentials(url, authenticator);

to

    if (allowAuthenticationReuse && !authenticator->isNull())
        authenticationManager->cacheCredentials(url, authenticator);

Another solution could be to add the check for is null in QNetworkAccessAuthenticationManager::cacheCredentials :

void QNetworkAccessAuthenticationManager::cacheCredentials(const QUrl &url,
                                                  const QAuthenticator *authenticator)
{
    Q_ASSERT(authenticator);
    if(authenticator->isNull()){
        return;
    }

These are untested pseudo-code