Well, tanks for your answer. For me, by adding this to the css file of my navbar, it don't affect anything. My Navbar html code:

  <li class="nav-item" routerLinkActive="active">
    <a class="nav-link waves-light" routerLink="leistungen" mdbWavesEffect>Leistungen</a>
  </li>

  <li class="nav-item" routerLinkActive="active">
    <a class="nav-link waves-light" routerLink="wohngemeinschaften" mdbWavesEffect>Wohngemeinschaften für Senioren</a>
  </li>

  <li class="nav-item" routerLinkActive="active">
    <a class="nav-link waves-light" routerLink="about" mdbWavesEffect>Über uns</a>
  </li>

  <li class="nav-item" routerLinkActive="active">
    <a class="nav-link waves-light" routerLink="aktuelles" mdbWavesEffect>Aktuelles</a>
  </li>
</ul>
<ul class="navbar-nav ml-auto nav-flex-icons">
  <li class="nav-item" (click)="openContact()">
    <a class="nav-link">
      <mdb-icon fas icon="envelope"></mdb-icon>Kontakt</a>
  </li>
  <li class="nav-item" (click)="openContact()">
    <a class="nav-link">
      <mdb-icon fas icon="align-justify"></mdb-icon>Impressum</a>
  </li>
</ul>

An earlier version of my code can also be found on Github, https://github.com/ProxieTV/dreitoechter-web What could be the problem?

You should use only .navbar-expand-lg selector in this case and set higher padding (because 0.5rem is already used and you won't see a difference). There are also some problems with the syntax of your code, here is a working example:

@media (min-width: 992px) {
  .navbar-expand-lg {
    padding-right: 5rem;
    padding-left: 5rem;
  }
}