Upgrading from WP 3.4.2 to 3.5 and voila, no more lightbox functionality :(
I don't know if the problem is other plugin or theme related ( I use Atahualpa).
In search of an alternative, compared to 'Lightbox Plus' and 'WP jQuery Lightbox' it seemed 'Simple Lightbox' was the clear winner.
First: maintained shortly and thus WP 3.5 compatible;
and Second, but even more important: it seemed 'compatible' with Flexible's Lightbox image class setting ('lightbox'), hence I don't need to 're-set' every image on the website to a new lightbox-functionality!
Now even with keyboard-support (left, right and ESC)!!!
A big thanks for this nice plugin! Highly recommended!