I'm having the same issue
I'm on frontend, trying to edit an article and the lightbox popup goes blank

I had mod_roktabs installed. Removed it, but no luck. Still having the problem. How can I trace the error? I don't see anything suspicious on Firebug... weird

imgur.com/5zW4A.jpg

I'm having the same problem with K2 v2.4.1. I've just installed it and I get the blank screen while trying to open Add New Item window. The built-in Jooml! SEF and shSEF404 are disabled.

Any suggestions?

If I don't remember well, it is the component.php file located inside /templates/your_template/ folder. Grab another component.php from another template or google how should be this file filled. If I remember well, I had the component.php file with 0bytes size, grabbed another one, and solved.

try to uninstall rs_firewall if you have installed it.