Pop Up: OnlyMe Authentication Required

Hi people of WordPress!

Every time lately that I visit my website, I get a pop-up window that says:

Authentication Required. The server https://mywebsite.com:80 requires a username and password. The server says: OnlyMe. User Name; __ Password:___

I have a username and password that I type in that gets me in. But I can’t figure out how to stop it / what OnlyMe is — does anyone know?

The window only pops up when I’m logged into my WordPress account. So no one else sees this pop up window except for me, when I’m logged in. I have a MacBook Pro and always do the automatic software updates, so I doubt this is malware/a virus.

Thank you so much for any insight!