Laravel 5.5 Type error: Argument 1 passed to Illuminate\Auth\EloquentUserProvider::validateCredentials() must be an instance

I suppose that you required to add implements \Illuminate\Contracts\Auth\Authenticatable to your UserAdmin model class definition.

    class UserAdmin extends Model implements 
    \Illuminate\Contracts\Auth\Authenticatable

You must use Authenticatable in User model

for example:

use Illuminate\Foundation\Auth\User as Authenticatable;

class User extends Authenticatable
{
  //your code
}

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