The admin user can't login, though the editors can

Might sound basic, but have to ask:

1. Did you try clearing browser cache?
2. Did you try using different browser?

Try creating test admin user manually in the database, see if this user can login.
https://www.wpbeginner.com/wp-tutorials/how-to-add-an-admin-user-to-the-wordpress-database-via-mysql/

Helo Viktor, thanks for your response.

I have tried in several browsers (Chrome, Firefox and IE) and also in private modes and the error is the same.

I also tried to add a new administrator user, but the error continues.

Thanks again, best regards.

So if you disable all plugins and change theme to something else, it still persists?

Yes, I have disabled all the plugins and the theme, and the error persists. Only the editors users can login to the dashboard.

Thanks, best regards.