Dropdown Form doesn't work unless i delete all the content

This problem is driving me crazy, i have the following drop-down form: Sign in

User Name --> Password Sign in

For some reason the button when i click it, it doesn't do ANYTHING. I don't know how to debug to be honest if any action is triggered and can't be performed. But the odd thing is if i remove everything inside the form but the login button it works, like this:

Sign in

                                <div class="form-group">
                                   <button type="submit" class="btn btn-success btn-block">Sign in</button>
                                </div>
                            </form>
                        </div>
                    </div>
                </ul>
            </li>

So to be honest i have no idea what is going wrong, i tried a lot of things and nothing seems to be working. Any idea?

THanks. BTW im using Bootstrap and Jquery.