Notification icon missing

Despite the Notification app is enabled in its version 0.3.0, the "bell" icon is missing in the GUI and notifications are not displayed. However, the activities about files and comments are available.
Running 9.1.2 on apache2 on Ubuntu 14.

Does this happen on all browsers? Check also if extensions are blocking this icon (add-blocker, anti-virus).

Yes, this happens with any browser, without add-blocker or security solutions.
In the notification's github documentation, there's a way to display notifications for a user:

When I try to access "/ocs/v2.php/apps/notifications/api/v1/notifications", I'm asked for credentials even if i'm already logged in OC.

Could it be some kind of permissions related issue?

No it shouldn't. This is an api, it can be used from external applications which then need authentication.

Can you open the browser error console if there is something wrong with some of the code?

Thank you for your help tflidd,
using the console I can see that the notification div is using class "notifications hidden":

<div class="notifications hidden"><div class="notifications-button menutoggle"><img class="svg" alt="Notifications" src="/apps/notifications/img/notifications.svg"></div><div class="notification-container menu"><div class="emptycontent"><h2>No notifications</h2></div><div class="notification-wrapper"></div></div></div>

In this css class, the parameter display is se to none:

.hidden {
display: none;

If I disable this setting then the notification bell appears.

So what is telling OC to use this style?

No idea what wrong here, it's probably the best to report this directly to the bugtracker:

I think it's difficult to arrange the icon within due to which there are chances to cause massive changes.

how are you creating notifications which shall be displayed?
e.g. the announcement center requires the cron job to run so that announcments are distributed to the users