In the AMP Beta 'TN/SR5' 18.104.22.168 Release Notes it says that you need to run the command below to reset the password for the Admin account.
AMP.exe -setpass NEWPASSWORD
Not having read the notice before upgrading, I ran into this issue but the above command did not work. Instead, I found a command for 1.7.0 in another post that did work for me (see below - replacing YOURUSERNAME with Admin in this case).
AMP.exe +Core.Login.Username YOURUSERNAME -setpass YOURPASSWORD -configonly
It might be a good idea if the AMP Instance Manager had a warning and a way to view the release notes before updating instances. That would prevent people like me that don't go checking documentation and just trust the software to update itself. :)
Thanks for a great product and the constant work you do to keep it updated!