Thank you very much, it works fine.

This is because during validation on the front-end, the input gets the status valid. then additional input styling works:

.form-control:valid {
    margin-bottom: 1rem;
    background-image: none;
    border-color: #00b74a;
}

The most important here is the additional margin. To reflect this, the easiest way is to add a margin to the message with feedback. Just add mt-1 class

  <div id="a_FK_HHC_Bilder04" class="invalid-feedback mt-1 d-block">
    Abteilung leer, bitte eingeben!
  </div>

I have made an example that works, but the error message should be a little lower, how does that work?

Please see snippet:

https://mdbootstrap.com/snippets/standard/gerdhuber/3303205

Unfortunately we do not have such an example. You have to create such validation on your own.