how can I force Joomla to show the Error Messages that a username or e-mail exists already in the Database while registrating...
When I try to register as a new user with an existing username or/and e-mail I'll be redirected to the completely filled out registration form without any error message or hint.
Thanks
By default Joomla shows errors messages for things like this. Whether they are displayed or not is down to your template. If you go to the following file:
templates/TEMPLATE_NAME/index.php
search for the following line which defines where Joomla messages are shown
<jdoc:include type="message" />
If you cannot find it, add it to where ever you like