Android - Gmail sign-in error

I have an older Android phone, HTC Inspire.

When I'm on wifi and I enter an ATT Hotspot, a white triangle icon will appear. The details say there's a Gmail sign-in error and a dialog box appears, showing my email address and a box for the password. It will NOT accept my password although I'm sure I typed it right. I can press Back and the box will go away, but the triangle icon will stay (unless I restart the phone). My real Gmail app will still accept my password.

I found some threads of people with the same prob and they said it has to do with Google+ updates, but I don't have that. Plus those threads are two years old.

This does not happen when I use wifi at other places, like the library.

Anybody know why this happens? It's shouldn't appear and doesn't even work. In the meantime I'm just going to keep wifi off unless I need to use it.

P.S. Some of the threads mentioned 2-step verification but I don't use that.
I have changed nothing lately and all of a sudden I started seeing this. I've been using the same phone for two years. Have not changed the Google password in a while, also.