Just a quick note (and possible cause).
You can have a perfectly correct VirtualHost
setup with _default_:443
etc. in your Apache .conf file.
But... If there is even one .conf file enabled with incorrect settings that also listens to port 443, then it will bring the whole SSL system down.
Therefore, if you are sure your .conf file is correct, try disabling the other site .conf files in sites-enabled
.