jQuery Colorbox randomly fails to group images

I use the jQuery Colorbox in my application, like so:

<ul>
    <li>
        <a href='images.html?cn=000001' >[000001] </a> - 
        <a href='/folder/1/Appreciation.jpeg'  class="imagelnk">Appreciation.jpeg</a>
    </li>
    <li>
        <a href='images.html?cn=000001' >[000001] </a> - 
        <a href='/folder/1/cheque.jpg'  class="imagelnk">cheque.jpg</a>
    </li>
</ul>

Over in javascript, inside .ready(), I have this:

    $('a.imagelnk').colorbox({
        rel: 'imagelnk',  
        current: '<i class="blue smaller">Document {current} of {total}</i>', 
        width: '1200px'
    });

Everything works okay, except that lately, I've noticed that every odd time, the lightbox fails to group. Therefore, only the very first a that is clicked gets loaded, and the current parameter above is totally missing.

This behaviour appears totally random, but frequent. There's no error message in the console.

My Colorbox version is 1.6.1, and jQuery version is jquery-2.1.3.

Please help, somebody!

EDIT

The error happens in Opera. I have just discovered that in Chrome, I cannot produce this error. For a certain reason I have to develop for Opera 12.17 (the latest "old" Opera before webkit) at the moment.

Does this work?

$('a.imagelnk').colorbox({
    rel: 'imagelnk',  
    current: '<i class="blue smaller">Document {current} of {total}</i>', 
    width: '1200px'
}).attr('rel', 'imagelnk');