After upgraded from 3.5.5 to 5.1.3 .. Channel Form doesn't return.. I tested with very simple code.. like
{exp:channel:form channel="form_test" return="inscription/merci"}
<div class="row" id="inscription">
<div class="column small-6">
<label for="nom">Nom <span class="required">*</span>
</label>
<input type="text" required="" name="title" id="title" value="" aria-required="true">
</div>
<button name="submit" type="submit" class="button expanded">SEND</button>
</div>
{/exp:channel:form}
I also tried to put these template in an other template group.. but i always get a white page after submiting.. Still the channel form is submited as i can see the new entry in CP..
But then i tried on an other EE install (other site with 5.1.3 from 4.x).. and it works!?