Built in administrator account

I've been using Windows 10 since the auto update from 8.1

Everything has worked since then, but then this happens.

Randomly in the last days, if I click on any app, I get prompted "App cannot be opened using the built-in administrator account."

I don't know where this is coming from. I don't use the login screen, it boots right to the admin, which is also the only account. I searched google and found not much useful.

I tried to add a user but it would not let me from the control panel. All that happened when clicking, is a console screen appears and disappears.

Is there any way to get around this? Why did it, after 6+ months, now show that message?

I tried setting the Microsoft login awhile back and it was a nightmare.

I've been able, and have been, using these apps for the last 6+ months without any issues. Suddenly last few days it prompts me.

My computers account has always been default. I only have this administrator account (with no password), so it auto logs me into the account when I boot.

I have also noticed, the last few days I haven't seen a UAC prompt either.