I went in and downloaded the latest Win64 bit version of the prealpha 39 last night. That was 15 hours ago, or 4/15/2018 about 9:00 pm EST.
Installed, etc. Was not able to use the Local Login nor was I able to telnet into the board.
Had to backtrack and go back to an older update.
Can you give any more detail? When you type "mystic" what happens? It should tell you exactly why its not working either directly on the screen or in the node log (but usually on screen).
I've seen this same type of thing reported over the years and usually when someone says this, its because they replaced .exe files but didn't follow the instructions to upgrade the prompts or data files...
Depending on what you're upgrading from you may need to do more than just "install replace c:\mystic" , and it seems like you might be coming up from an older version?
As you can see by my tagline and many others here, it is working for us.
--- Mystic BBS v1.12 A39 2018/04/14 (Windows/64)
* Origin: Sector 7 [Mystic BBS WHQ] (21:1/108)