Currently the HPS site is down, has been for some time.
According to a report I saw on another forum from Rich Hamilton they have a server problem and are working on it, not sure if he is still affiliated with HPS or not.
I do know that JTS has not provided HPS with the newest updates beginning in Jan. 2016 but they will help you update to the newer JTS updates.
These require a serial number to activate the game but if you go to
http://johntillersoftware.com/Support.php you can contact their support team.
They can assist you in migrating to the downloaded JTS versions of your games.
It may take a couple days to receive an answer but I have always gotten a reply when contacting them.
I was able to move to the new versions of all of my Tiller HPS games, 40+ titles from the ACW, Nappy, and PzC games at no cost to me.
I don't have all of them installed but I did not have any problems with my pc during or after installing the updates for the ones I do so I'm not sure what type of issue installing the update would produce other than requiring a sn to activate after the update.
I also had a problem with my HPS NRC disk not working and Rich provided me with a link to download the JTS installation file.
So while I don't always agree with everything they do IMO the support of older Tiller titles is better than anything I have seen in 30+ years of buying pc wargames.