What version of PHP should I be using?

Is there a solution to this?

This is it. For some reason, my build was missing username, so once I added it worked. Thanks!

I think you're right, but I have yet to find the answer.

Thanks Mohammad! Do you have examples of live sites?

Just checked it. It's not.

I did that but it doesn't work. And I also tried different themes too.

@blenddev What's your system setup? Do you have a VCS running and a deployment script? If you do, once you push the code up to production remember to run admin.php in production once again. It worked for me.

I decided to roll back and upgrade my site one more time and now it works. I guess the take-away here is if you version control your EE site, then once you run the installer locally, you need to run it on prod as well. Not sure if this was the cause of my issue but it works now.

Looking at it closely, I've never changed is_system_on to "n" when upgrading. What does that do?

In my experience, you need to run installer in prod even if you run installer locally.