Apache HTTPS Issues – Troubleshooting Apache Randomly Stopping HTTPS

We have this Apache on Windows Server 2012:

Server version: Apache/2.4.9 (Win32)
Apache Lounge VC10 Server built: Mar 17 2014 10:48:43

Most of the time, it works perfectly, but from time to time (randomly, cca 3-4 times per week), it stops serving HTTPS on port 443, but continues to serve HTTP on port 80. When it stops serving HTTPS, the only way to resolve the problem is restarting Apache.

This seems to be the very same problem as is here: Apache stops responding to http requests — https continues to work except that in our case it is HTTPS that stops working and HTTP is the one that works all the time.

We have enabled trace6 level of verbosity to get some good information on what is going on. However, the logs are empty:

...
[Sat Mar 21 07:51:50.577373 2015] [ssl:debug] [pid 3356:tid 2540] ssl_engine_io.c(999): [client ...:16529] AH02001: Connection closed to child 137 with standard shutdown (server ...:443)
[Sat Mar 21 07:54:21.936742 2015] [ssl:info] [pid 4760:tid 432] AH01914: Configuring server ...:443 for SSL protocol
...

In this log, we can see the last line for the last request that was served OK at 07:51:50 and then there is nothing (our internal monitoring system attempts to connect every minute and so there should be 07:52:50 record, but it is missing). The next line at 07:54:21 is after our internal monitoring system timed out and restarted the Apache service.

Our internal monitoring is in C# and its output was:

System.Net.WebException: The operation has timed out
at System.Net.HttpWebRequest.GetResponse()
at ExecuteServicePageCheck(Object stateInfo)

Our httpd-ssl.conf looks like this:

  <VirtualHost _default_:443>
        ServerName ...:443
        ServerAlias www.....com
        DocumentRoot ${US_ROOTF_WWW}/.../www/www
        SSLEngine On
        SSLProtocol All -SSLv2 -SSLv3
        SSLHonorCipherOrder on
        # Prefer PFS, allow TLS, avoid SSL, for IE8 on XP still allow 3DES
        SSLCipherSuite "EECDH+ECDSA+AESGCM EECDH+aRSA+AESGCM EECDH+ECDSA+SHA384 EECDH+ECDSA+SHA256 EECDH+aRSA+SHA384 EECDH+aRSA+SHA256 EECDH+AESGCM EECDH EDH+AESGCM EDH+aRSA HIGH !MEDIUM !LOW !aNULL !eNULL !LOW !RC4 !MD5 !EXP !PSK !SRP !DSS"
        # Prevent CRIME/BREACH compression attacks
        SSLCompression Off
        # Commit to HTTPS only traffic for at least 180 days
        Header add Strict-Transport-Security "max-age=15552000"
        SSLCertificateFile ${US_ROOTF}/core/apache2/server_certs/....crt
        SSLCertificateKeyFile ${US_ROOTF}/core/apache2/server_certs/....key
        SSLCertificateChainFile ${US_ROOTF}/core/apache2/server_certs/....ca-bundle
        CustomLog "logs/.../www/access.ssl.%Y.%m.%d.log" combined
        ErrorLog "logs/.../www/error.ssl.log"
        LogLevel trace6
    </VirtualHost>

Is there anything more we could do to get any information on what is going on there? We have no error message from Apache anywhere, not even on this trace6 level.

Note that we do have more domains served on both HTTP and HTTPS and the problem occurs for all of them on HTTPS. It is like if Apache just closed that port silently.