I changed something in the html code. Which (I’m really sure) also did yesterday but now it is working
The working html code for a working POST method looks like following:
<div class="register-photo" style="height: 80vh;background-image: url("https://files.onlosant.com/5cd81d7871914b0007d1795b/webiste_files/190113_career.jpg");">
<div class="form-container">
<form id="form-login" method="post">
<h2 class="text-center"><strong>Create</strong> an account.</h2>
<div class="form-group" id="firstname">
<input autofocus minlength="1" maxlength="255" value="{{ pageData.firstName }}" type="text" class="form-control" name="firstName" id="firstName" placeholder="e.g. Joe">
</div>
<div class="form-group" id="surname">
<input autofocus minlength="1" maxlength="255" required value="{{ pageData.lastName }}" type="text" class="form-control" name="lastName" id="lastName" placeholder="e.g. Doe">
</div>
<div class="form-group" id="usergroup">
<input required value="{{ pageData.userGroup }}" type="object" class="form-control" name="userGroup" id="userGroup" placeholder="instagrid">
</div>
<div class="form-group" id="email">
<input required value="{{ pageData.email }}" type="email" class="form-control" name="email" id="email" placeholder="e.g. max.mustermann@example.com">
</div>
<div class="form-group" id="password">
<input required minlength="8" maxlength="255" type="password" class="form-control" id="password" name="password" placeholder="At least 8 characters">
</div>
<div class="form-group"><button class="btn btn-primary btn-block" id="button-sign-up" type="submit">Sign Up</button></div><a class="already" href="/login">You already have an account? Login here.</a></form>
</div>
</div>
Thanks for the help