I am running latest version of K2 and Joomla 2.5.9.
I tried to change the author of an item. When clicking change a window opens with all the users. When clicking on a user nothing happens and I get the following error in firebug

TypeError: window.parent.SqueezeBox is undefined
[Break On This Error]

if ( typeof (window.parent.SqueezeBox.close == 'function'))

I know this is not a new error so is anything being done about it. There doesn't seem to be an obvious solution. I tried the suggestion of removing blank users, but that had no effect.

Hello John,

This is a JS error so it could be caused from a lot of things, firstly your editor, is it up to date?

Make sure that jQuery is not loaded twice from a 3rd party component and also make sure that the version of jQuery or MooTools that is loaded is compatible with other components or plugins you have installed.