I was knocking my head into a wall trying to get the SQL root password changed, and I came across several threads on various pages of the forums here - all instructing me on what to do, which files to change, etc. I'm running 8.9.2, but no matter which methods I tried, I couldn't get it working correctly.
I tried a side install using XAMPP on another system and it seemed to be fine, but I noticed that the password I generated for the SQL install was noticably shorter than the one I used for Uniform Server.
I can't seem to find anything on your documentation that states this, but for those of you out there using password managers like Lastpass and KeePass, be sure you're not generating passwords that are too long and too complex - I was generating a 32 character password with special characters and numbers, and it refused to let me into phpmyadmin no matter how many times I restarted the service/servers.
Once I changed the password to something a little shorter, it worked without a problem.
If this is documented somewhere, I apologize - I couldn't find it, so I hope this helps someone out if it isn't out there.
Great package! Long time XAMPP user, looking forward to giving this a full go.