Why am I having trouble entering a new Gmail password in Mail (and Internet Accounts)? I'm required to change my school Gmail password every year.  I recently did so, and it seems to work just fine for accessing Gmail with a web browser.  It also works fine in the Mail app in iOS (I was asked to enter the new password, but it accepted it just fine).  However, in the Mail app on macOS, I've run into some issues.  It shows a "Login Failed" message on the top right of the window as well as the telltale lightning bolt "offline" icon to the right of the mail in the left sidebar.  I totally expected to see both of those, since Mail is trying to access the account using the old password.  But I can't get past that.  If I click on either of those, it opens a dropdown/popup where I can enter the password:   That's all good.  After I enter the password, though, I see a "loading" animation (a spinning-type one) for a brief second and then just blank gray in the window:   The same thing happens if I try to do this in the Internet Accounts section of System Preferences.

Does anyone know why this is happening and how I can resolve it?
I had this issue recently. There are some answers here, but what worked for me was I had to sign out of my iCloud account first. Add/Update the Google password (should now work) then sign back into iCloud.