Stuck with ->back() and session message not showing up

Hello,

I have been for a couple of hours on this one and cannot find an answer to it.

    public function update(Request $request, $id)
    {

        //Find user details
        $findUser = DB::table('users')->where('id', '=', $id)->first();

        $this->validate(request(), [
        'name'     => 'required|string|min:2|max:64',
        'surname'  => 'required|string|min:2|max:64',
        'username' => 'required|string|min:2|max:64',
        'email'    => 'required|string|email|max:128',
        'town_id'  => 'required|integer',
        'password' => 'required|string|min:6'
        ]);


        //If username from the form is not the same as username from database
        if($findUser->username != $request->get('username')){


        //Check if username exists in the whole database
        $usernameExists = DB::table('users')->where('username', '=', $request->get('username'))->first();



        //If username does not exists
        if(!$usernameExists){

        //Add this new username to user profile
        $updateUsername = DB::table('users')->where('id', '=', $id)
            ->update(array('username' => $request->get('username')));

        return back()->with('success', 'Username updated!');

        }else{


        return back()->with('errors', 'Sorry but this username has already been taken!');

        }
        }
        }

I am on the edit page of a user's profile

When the username does not exists, the message:

      return back()->with('success', 'Username updated!'); 

shows up correctly on return.

But when the user exists, this session:

   return back()->with('errors', 'Sorry but this username has already been taken!');   

Causes the error:

ErrorException (E_ERROR)
Call to a member function has() on string (View: C:\laragon\www\myproject\resources\views\admin\users-edit.blade.php)

The weird thing is that both the error and success pages are identical:

Partial success

@if (session()->has('success'))
    <div class="alert alert-success text-center animated fadeIn">
        <button type="button" class="close" data-dismiss="alert" aria-label="Close">
            <span aria-hidden="true">&times;</span>
        </button>
        <strong>
            {!! session()->get('success') !!}
        </strong>
    </div>
@endif 

Partial error:

@if (session()->has('errors'))
   <div class="alert alert-danger animated fadeIn text-center">
        <button type="button" class="close" data-dismiss="alert" aria-label="Close">
            <span aria-hidden="true">&times;</span>
        </button>
        <strong>
            {!! session()->get('errors') !!}
        </strong>
    </div>
@endif

Any idea why the success works and not the error partial?

Thank you so much!