Custom Font not displayed

Dear Community,

I decided to switch my Watchface to a custom Font, but once I did that, I only see "placeholders" (rectangles filled with background color of the font) and no characters. Switching back to system font works. To make sure it's not the Font itself, I tried the Black Diamond Font from the Analog sample.

Here is the code:

using Toybox.WatchUi;
using Toybox.Graphics;
using Toybox.System;
using Toybox.Lang;
using Toybox.Time;
using Toybox.Time.Gregorian;
using Toybox.Math;
using Toybox.ActivityMonitor;

...

function onLayout(dc) {
font = WatchUi.loadResource(Rez.Fonts.blackdiamond);

...

dc.setColor(0xFFFFFF, 0x000000);
dc.drawText(center, center, font, now.sec, 5);

And the resources.xml:

<resources>
    <fonts>
        <font id="blackdiamond" filename="fonts/blackdiamond.fnt" antialias="true" filter="0123456789"/>
    </fonts>
</resources>

Am I missing something?