Certificate problem?
Off my hat I would say the one working site has tried to implement SSL. This requires a use of a dedicated IP address. If your server has only one IP address then what would the rest of the sites use? Now days I always use at least two IP's per server. One for the server, one for the sites (and more for possible SSL's.)
You should check out if this will happen again when someone tries to add a new SSL Certificate.
|