amurs

i have the same problem and in the same line
i tried deactivating all the plugins, uploading core files and changing the theme but the problem is still there
I think one on the db tables is corrupt

thank you @bcworkz

I use a www.ads-software.com self hosted website with a custom domain and i use Jetpack plugin (by Automattic) which offers this option of logging in to wordpress via wordpress.com account.

I found the solution here

Disable Default Login Form
If you’d like to completely disable and hide the default login form, and force users to log in via WordPress.com, you can use the following line of code:

add_filter( ‘jetpack_remove_login_form’, ‘__return_true’ );