Error htmlspecialchars () in Laravel 5.5. Error with sessions

I am creating a website of job offers in Laravel 5.5, and when trying to implement a functionality that allows you to add sectors (for example, IT and Hospitality) to filter the offers for those sectors, from time to time I get the error

  

htmlspecialchars () expects parameter 1 to be string, array given   (View:   C: \ xampp \ htdocs \ bagsemployment \ resources \ views \ offers \ index.blade.php)

To temporarily "fix" that error I have to log out of the logged-in user and restart it.

This is my code:

SectorController.php

public function arraySectors(Request $request) {

      // Generar Array
      if($request->sector == null || $request->sector == "") {
        return back()->with('message', ['danger', __('Por favor selecciona un Sector')]);

      } else {
        $array[] = $request->session()->get('sectors', 'sectors');
        $datos = array_merge($array);
        $request->session()->put('sectors', $datos);

        if(count($request->session()->get('sectors')) >= 5) {
          return back()->with('message', ['warning', __('Sólo se pueden seleccionar 5 sectores al mismo tiempo')]);

        } else {
          $request->session()->push('sectors', $request->sector);
          $request->session()->save();
          $sectors = session('sectors');

          // Consulta
          $array = array_values($sectors);
          $nOfertas = Oferta::whereIn('sector', $array)->latest()->paginate(5);

          return back()->with(compact('sectors', 'nOfertas'));
        }
      }


    }

index.blade.php

<tbody>
  @if(!isset($nOfertas))
    @forelse($ofertas as $oferta)
    <tr>
      <td class="py-3">{{ $oferta->titulo }}</td>
      <td class="py-3">{{ $oferta->descripcion }}</td>
      <td class="py-3">{{ $oferta->empresa }}</td>
      <td class="py-3">{{ $oferta->sector }}</td>
      <td class="py-3">{{ $oferta->fecha_limite }}</td>
      <td class="py-3">
        <form class="form-horizontal" method="POST" action="offers/{{ $oferta->id }}/register">
          {{ csrf_field() }}

          <input type="hidden" name="id_user" value="{{ Auth::user()->id }}">
          <input type="hidden" name="id_oferta" value="{{ $oferta->id }}">
          <button type="submit" name="registerOferta" class="btn btn-primary">
            Inscribirse
          </button>
        </form>
      </td>
    </tr>
    @empty
    <p>
      No hay ofertas actualmente
    </p>
    @endforelse

  @else
  @forelse($nOfertas as $oferta)
  <tr>
    <td class="py-3">{{ $oferta->titulo }}</td>
    <td class="py-3">{{ $oferta->descripcion }}</td>
    <td class="py-3">{{ $oferta->empresa }}</td>
    <td class="py-3">{{ $oferta->sector }}</td>
    <td class="py-3">{{ $oferta->fecha_limite }}</td>
    <td class="py-3">
      <form class="form-horizontal" method="POST" action="offers/{{ $oferta->id }}/register">
        {{ csrf_field() }}

        <input type="hidden" name="id_user" value="{{ Auth::user()->id }}">
        <input type="hidden" name="id_oferta" value="{{ $oferta->id }}">
        <button type="submit" name="registerOferta" class="btn btn-primary">
          Inscribirse
        </button>
      </form>
    </td>
  </tr>
  @empty
  <p>
    No hay ofertas actualmente
  </p>
  @endforelse
  @endif
</tbody>

web.php

Route::post('/offers/add_sector', 'SectorController@arraySectors');