PC, Windows 10, the latest Acrobat Pro, updated.
I tried saving a doc under a different name and the pop up window for "save as" did open, but all I was seeing was the "box" trying to load, without much success. Eventually I gave up, copied the doc, and renamed it, edited it and then saved it on top of its old self.
But the fact remains that the "save as" does not work on my computer.
I am sorry for late response.
You might try the steps given in the following link which discusses similar issue:
Let me know if this is not the case.
Are you still facing this issue?
If the steps given in the above link doesn't work, you might try to install the latest update which has been released recently.
Launch Acrobat>navigate to Help menu>select Check for Updates.